The demodulation method of active homodyne laser nanometer vibration measurement system based on digital signal processing (DSP) system is presented. In this paper, the software and hardware designs of low-frequency nanometer vibration measurement system is demonstrated based on the technology of phase generated carrier (PGC). The key technique of PGC based on DSP includes the following aspects: the choice of carrier frequency and the modulation depth of phase are expounded for the need of high sensitivity detection and large dynamic range; the designs of sampling frequency and anti-aliasing filter are introduced. Experiment result indicates that using digital signal processor chip as the core signal processing component has more advantages than analog system: the intrinsic direct current offset from temperature shifting of analog PGC circuit is eliminated; the disturbance of low frequency noise is suppressed; the systemic adaptability and the ability of anti-jamming are improved; besides, this system is easy to be miniaturization. This detecting system can be used for measuring weak vibration of nanometer measurement with frequency ranging from 10 to 100Hz.The result shows that amplitude-detecting resolving power is greater than 0.1nm(value of peak-peak) and the largest detectable amplitude is 10μm. It is a promising practical demodulation scheme for low frequency weak vibration measurement.