Abstract: Abstract: Irrigation requirement is important for agricultural water management and irrigation decision making, and it is strongly influenced by interannual fluctuations of rainfall and evapotranspiration. The summer maize is one of the main crops in the China North plain, and can be greatly affected by annual variation of rainfall and potential evapotranspiration during its growing season. For that reason, supplementary irrigation estimated based on the rainfall and potential evapotranspiration is necessary. In this study, the summer maize was selected as a research object in the southern area of the China North Plain, and the analytic relation between irrigation requirement and rainfall parameters associated with potential evapotranspiration was described on the basis of probability density function of soil moisture in order to investigate the influence of rainfall and potential evapotranspiration on irrigation requirement. The annual variations and statistical characteristics of rainfall, potential evapotranspiration and irrigation requirement were analyzed firstly, and then the Monte Carlo method was employed to investigate the long-term influences of rainfall parameters and potential evapotranspiration on irrigation requirement. The long series of rainfall parameters and potential evapotranspiration were generated by a pseudo-random number generator according to their probability density functions. The standard deviation and variation coefficient of irrigation requirement were adopted to assess the contribution of rainfall parameters and potential evapotranspiration to irrigation requirement fluctuations. The results showed that the annual variability of rainfall parameters was remarkable. Rainfall mean depth fluctuated between 4.958 and 25.003 mm with a variation coefficient of 0.326, and rainfall frequency fluctuated between 0.143 and 0.457 d-1 with a variation coefficient of 0.170. Rainfall mean depth conformed to a Logistic distribution with the expected value of 11.273 and the standard deviation of 2.022, and rainfall frequency conformed to the same distribution but with the expected value of 0.318 and the standard deviation of 0.029. Both rainfall mean depth and rainfall frequency presented an unmarked downward trend by Mann-Kendall tend test. While the annual fluctuation of potential evapotranspiration, which subjected to a LogNormal distribution with the expected value of 1.370 and the standard deviation 0.076, was smooth and steady, and there was a significant downward trend for it. Irrigation requirement fluctuated between 8.1 and 381.8 mm around the average value (133.1 mm) with a variation coefficient of 0.673, and the variation was significantly larger than rainfall parameters and potential evapotranspiration. Irrigation requirement conformed to a Gamma distribution with the shape parameter of 2.173 and the scale parameter of 61.271. There was an unremarkable upward trend for irrigation requirement. When taking no account of the annual fluctuations of rainfall parameters and potential evapotranspiration, the calculated average value of irrigation requirement was 114.3 mm. It indicated that it would underestimate the annual average value of the irrigation requirement without consideration the annual fluctuations of rainfall parameters and potential evapotranspiration. When only rainfall mean depth was considered as random variable, the standard deviation and variation coefficient of irrigation requirement were 63.6 mm and 0.485. When only rainfall frequency was considered as random variable, the standard deviation and variation coefficient were 41.9 mm and 0.354. When only potential evapotranspiration was considered as random variable, the standard deviation and variation coefficient were 28.4 mm and 0.245. While all of them were considered as random variables, the standard deviation and variation coefficient were 88.5 mm and 0.651. Rainfall mean depth contributed to the largest share of irrigation requirement fluctuations, followed by rainfall frequency and potential evapotranspiration. Annual fluctuations of irrigation requirement presented a larger uncertainty under the mutual effects of rainfall parameters and potential evapotranspiration. Consequently, it is necessary to assess the annual uncertainty of irrigation requirement, and to develop a reasonable strategy according to different rainfall and evapotranspiration conditions in order to reduce the negative impacts of climate fluctuations and guarantee food safety in China.