Abstract: A target monitoring system based on the wireless sensor network works by precepting the changes of the magnetic field, the vibration, the sound and other kinds of signals. In view of the constrained energy supply for the wireless network, a low-power design is proposed, which includes the selection of the sensor chips, the dormancy design of nodes, the determination of the network topology, and the data transmission protocol. It is shown that the average working current of the sensor system is only 3.78 mA with a configuration of 4 A·h lithium battery and a sampling rate of 10 Hz for each node, to be run continuously for 33 days.