Abstract: Abstract: Subsurface pipe drainage has been the main engineering measure for saline-alkali land improvement, due to the easily mechanized operation, better control of saline-alkali soil without occupying cultivated lands. Two- or multi-stage pipe drainage system has been also widely used for easy maintenance and management in recent years. A photovoltaic power generation can serve as the power supply source of pipe drainage pump stations, particularly for simple and cost-saving. However, the solar photovoltaic water pump cannot operate smoothly, when the outlet of pipe drainage is submerged by a high water level at night. The capacity of the system can be greatly limited even to a complete loss. In this study, an improved capacity of drainage system was proposed for the solar photovoltaic subsurface pipe system in Liuzhong Town, Pingluo County, Yinbei irrigation area, in China. The feasibility of siphon auxiliary drainage was analyzed to determine the drainage capacity of the current system using the formula of siphon flow. Some parameters were then monitored to verify the improvement of drainage capacity, including the regional drainage discharge, the water level in the sump and drainage ditch, and groundwater level. The results show that there was a limited drainage capacity of the previous solar photovoltaic subsurface pipe system, where the drainage rate was less than or equal to 0.49 mm/d, far less than the drainage rate of 1.5 mm/d required by the standard control of salinization. Since the water level rose in the manhole at night, while the outlet of the subsurface pipe was submerged, the water level in the sump was restored to close to the groundwater level and higher than the water level of the adjacent drainage ditch. The positive difference in the water level between sump and ditch was more than 0.3 m. Alternatively, the siphon was selected for the auxiliary drainage at night, so that the drainage efficiency was greatly increased in the improved system. Since the diameter of the siphon was about 0.09 m, the drainage discharge at night was equivalent to that of the solar photovoltaic pump in the daytime. A vacuum pump and a few siphon pipes were added, indicating the low investment cost and being easy to implement. A horizontal connecting pipe between the sump and the adjacent drainage ditch was also added to realize the gravity drainage at night during construction. The findings can provide technical support for the application of solar photovoltaic subsurface pipe drainage systems.