In order to measure soil moisture automatically and realize wireless transmitting for the results, electric field method was used to detect the water content in soil, and wireless sensor network was built with Zigbee technology. With solar cell, solar energy was collected and stored in Li-ion battery to provide power supply for the system. Through reasonable charge-discharge management, the life of Li-ion battery could be prolonged. Experiment results showed that only soil moisture range from 0 to 30% could be detected, and the relative error was less than 10%. With Zigbee module and MiWi (TM) protocol stack, a star topologic wireless sensor network was built, and the results could be transmitted via wireless network. When the transmitting power of sensor node was set to 1mW and there was no barrier between sink node and sensor node, the valid transmission distance was 30 miters, if there were crops between sink node and sensor node, the distance was 10 miters. If datas were acquired and transmitted once an hour and the weather was normal, the system could work continuously. Even if the weather was overcast or rainy for a period, the equipment could work normally for 7 to 20 days.