Abstract: The effect of water supply on the grain filling process of crops by constructing models has been intensively studied, while there are few reports on the fitting analysis of the grain filling process of summer maize under different irrigation amount and types of nitrogen fertilizer synergists. In order to reveal the effect of irrigation amount and nitrogen fertilizer synergists on the grain filling process of summer maize, the maize variety Zhendan 958 was selected in a field experiment in Guanzhong area, Shaanxi Province, China. Two levels of irrigation amount (i.e., low water W1, 40 mm; high water W2, 60 mm) and three types of nitrogen-based synergists were added on the basis of single urea application(N-(n-butyl) thiophosphoric triamide, Dicyandiamide, N-(n-butyl) thiophosphoric triamide+Dicyandiamide) were set in the experiment, thus the experiment contained 8 treatments. The grain filling process model affected by irrigation amount and nitrogen fertilizer synergists was established and verified based on Richards equation, and the filling characteristic parameters including initial growth potential, grain filling active days, occurrence time of the maximum grain-filling rate, maximum grain-filling rate and mean grain-filling rate were analysed. The effects of irrigation amount and nitrogen fertilizer synergists on grain-filling duration days and rates in different stages, dry matter transfer quantity and rate, and yield were also analysed. The results showed that Richards model could better simulate the dynamic process of grain filling in summer maize, with determination coefficients of the grain filling fitting equation for each treatment above 0.99. Under irrigation amount level W2, nitrogen fertilizer synergist Dicyandiamide increased the seed initial growth potential; N-(n-butyl) thiophosphoric triamide+Dicyandiamide extended the period of grain filling active days, retarded maximum grain-filling rate and possessed higher maximum grain-filling rate and mean grain-filling rate. In relation to low water W1, high water W2 significantly retarded maximum grain-filling rate and prolonged the total grain-filling duration days, and improved mean grain-filling rate at grain-filling slowly increase period. Dicyandiamide and N-(n-butyl) thiophosphoric triamide+Dicyandiamide significantly improved the mean grain-filling rateduring grin-filling fast increase period and slowly increase period. The interaction of irrigation amount and nitrogen fertilizer synergist on grain-filling duration days, mean grain-filling rate and other parameters were also significant. Under high water W2, Dicyandiamide could significantly extended grain-filling duration days to 47.10 d, meanwhile optimized the proportion of the three grain-filling stages with relatively higher mean grain-filling rateof 0.828 g/d. The transfer quantity and contribution rate to grain stored in stems and sheaths were higher than that in leave. Compared to low water W1, high water W2 possessed 3.63% lower contribution rate of stem and sheath to grain. For the perspective of nitrogen fertilizer synergists, N-(n-butyl) thiophosphoric triamide+Dicyandiamide obtained the highest contribution rates of stems and sheath to grain, followed by N-(n-butyl) thiophosphoric triamide, Dicyandiamide and single urea. The grain yield of summer maize under the three nitrogenous synergists was significantly higher than that of single urea, respectively; showing yield obtained by Dicyandiamide and N-(n-butyl) thiophosphoric triamide+Dicyandiamide were higher than N-(n-butyl) thiophosphoric triamide. Increasing the amount of irrigation to 60 mm can enhance the yield increase effect of nitrogen fertilizer synergist, the yield of N-(n-butyl) thiophosphoric triamide, Dicyandiamide and N-(n-butyl) thiophosphoric triamide+Dicyandiamide increased by 6.55%, 5.36% and 10.53%, respectively.Overall, both increased irrigation amount and nitrogen synergists could improve summer maize yield, and higher irrigation amount improved maize yield by prolonging grain filling duration days. High water W2+Dicyandiamide was recommended as the optimized combination of irrigation amount and nitrogen fertilizer synergist in this experiment as it extended the total grain-filling period of summer maize, and benefit for yield formation. These findings could provide novel knowledge for the optimization of irrigation amount and nitrogen fertilizer synergist in the production of summer maize.