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Photovoltaic (PV) water pumping system has been proved being environmental-friendly and low energy-cost, which has a promising prospect in arid areas which are rich in solar energy resources. However, water resources remain to be one of the main restraints to the application of PV water pumping system widely. Models to evaluate the evapotranspiration and groundwater level in the well are estimated based on the data of a field trip, which was conducted in Wulanchabu grassland, Inner Mongolia, China. Data about the performance of the PV water pumping system, including evapotranspiration and the groundwater level variation were collected. In this paper, evapotranspiration is calculated by Penman-Monteith method and Theis formula is introduced to calculate the soil characters and simulate the groundwater level variation. PRMSE and Nash-Sutcliffe efficiency are used to validate the model performance with the collected lysimeter data and groundwater level. The results show that the modeling of the evapotranspiration and groundwater level is reliable. According to the water demand and energy demand, the method to optimize the pumping system is introduced. Based on the calculated result and collected data of water demand and groundwater level, groundwater in the site is abundant to support the system. However, due to the stickiness of the soil condition, the recharge rate is smaller than the pumping rate. According to the comparison, the current system is oversized. In this condition, the pump would run out of water, and have to stop pumping. Therefore, not only the water quantity is an important factor to be considered, the soil condition and recharge rate are also of great significance to the optimization of photovoltaic water pumping systems.