Collecting field soil moisture information is the foundation of auto-irrigation. This paper introduced a soil moisture information collecting system based on wireless sensor network (WSN) technology and with application background of automatic drip irrigation for cotton field. Firstly, application background was analyzed and application requirement was defined. The system worked together with a drip irrigation system in cotton field. After study, it was found that the output of soil moisture sensor in a certain size of area was very nearly, which leaded to the possibility of network data compression. In the second part, the frame of the system was given. It was composed of battery-powered field sensor nodes, solar powered WSN-GPRS gateway and data manage server. In the third part, hardware and software of the sensor node were introduced in details. Especially, a novel communication protocols was proposed. In the protocols, all the sensors were encoded with Morton code and a logical multi-level cluster was constructed among the nodes. Making use of the similarity of the output of the sensors, the protocols reduce power cost significantly and prolong life of the network. More sensor nodes had little impact on the network because Greedy Perimeter Stateless Routing (GPSR) algorithm was used to routing data. The protocols can also support sensor node to enter network sleep mode. At last, some test and simulation results were shown in diagrammatic form. The system was field tested over six-month period (April 2009 to Oct. 2009) in Xin-jiang province of China. The experimental results demonstrated that large scale, long-term collecting of soil moisture information can be achieved by using our proposed system.