With the aim to raise measure accuracy of double-depth measuring water content tactics in precision irrigation control system, by integration research of water content sensor, dual-layer composite soil water content sensor was worked out. Compared with the traditional two sensors’ tactics, using dual-layer composite soil water content sensor, the measurement result can be more accurate, and energy consumption was reduced. In order to achieve optimal irrigation amount of green land plant, ensure the irrigation not be wasted and satisfy plant’s normal need, the experiments were carried out by taking tall fescue for an example. On the basis of aggregately analyzing the tall fescue’s growth requirement, soil water content and parts of meteorological dates, the result was gained, that is the optimal irrigation amount of tall fescue is 12mm.