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In light of the requirement of water-saving irrigation in precision agriculture, a measuring system was developed to determine soil water content and the position of sampling sites. The system mainly consists of a soil water content sensor, a GPS receiver, and a portable instrument with RS-232C serial communication port. The design of the instrument, receiving of GPS data, and principle of water sensor were introduced. Tests were conducted to estimate the performance of the instrument and the accuracy of GPS. The data were processed by GIS software ARCVIEW 3.3. Results showed the MAP330 GPS receiver could meet the requirement of field positioning, and the distribution map of soil water could provide a scientific basis for precision irrigation.