The design and implementation of an indoor location system based on wireless sensor networks(WSN) and ultrasonic are studied.The distance between the locating object and the anchor node is calculated using time of flight(ToF) and velocity of ultrasonic.Then the trilateration localization is applied to compute coordinates.The envelope detection method is used to measure ToF, which treats the highest value point of the envelope signal as the arrival time.Considering the effect of noise in the actual circuit, the measured value will fluctuate at the actual peak, it affects the stability and precision of the system.The digital lock-in amplifying technique is adopted in this paper, the result of experiment and simulation show that this method can reduce the effects of noise and interference, and improve greatly the positioning accuracy and the capacity of anti-jamming compared with the existing methods.Without considering the ultrasonic radiation, the distance measurement error is within 2mm, and the standard deviation is less than 0.15mm in the experiments, which is reduced about 50% comparing with the best result achieved currently 0.3mm.