Based on wavelength modulation spectroscopy technology,a phase locked amplifier for the second harmonic signal is designed,which used the keying method for phase sensitive detection. Compared with the traditional phase sensitive detection method,the keying method simplifies the design of the circuit and reduces the cost of the lock in amplifier. In this paper,MATLAB software was used to build the simulation model,including band pass filter,phase sensitive detector,low pass filter and so on. When the second harmonic signal amplitude is 0.1 V and the Gaussian band limited white noise variance is 10,the maximum output error is less than 2% and has good linearity. The simulation results show that the method can be used for gas concentration detection,and the method is simple and cost effective.