Abstract: To improve the specifications of seismometers,laser Doppler measurement technique and weak signal detect technology and etc are studied,and a differential Doppler demodulation system with a LD is designed.In this system,more Doppler signals are received and more space resolving power are achieved because of adopting Gaussian beams focus and lens with large diameter as well as acoustooptic modulation technology and lock-in amplifier used to improve SNR.It has characteristics of simple structure,fixing and adjusting convenience,high use-ratio of light power and space resolution.The measurable ranges of frequency and amplitude are 0.5Hz～1000Hz, 2μm～5mm respectively,and the corresponding measurement precisions are 0.6%,1% respectively.