In order to precisely determine the extent of the water deficit and thus to provide a scientific reference for precision irrigation, a particular design of farmland surface humidity monitoring system based on wireless sensor networks is introduced. According to the features of XinJiang agricultural environment，a two-layer deployment architecture is proposed to satisfy the need of low-cost，long lifetime and large scale in this paper at first. In architecture the bottom layer is deployed evenly to monitor the information of soil moisture precisely and the cluster head nodes is distributed unevenly based on the number of packet transmitted and the distance to sink to ensure the cluster heads energy consumed balancedly. By the simulation and comparing to the leach-c and leach, it shows that the aims of scarcely deployment，energy-saving, equilibrium of energy consume and prolonging the system lifecycle is achieved.