The amplitude and waveform of self-mixing interference signal change periodically with static length of external cavity. The changes of the amplitude and waveform are measured experimentally with variation of static length of external cavity. And,the variation of the amplitude with static length of external cavity is calculated by superposing of self-mixing interference signal of multi-longitudinal mode laser. Then,that variation of the waveform is analyzed by considering the effect of feedback light on reflectivity of laser-cavity and laser medium gain coefficient. Results show that,the variation period of self-mixing interference signal of multi-longitudinal mode laser with static length of external cavity is equal to optical length of laser cavity. While,the width of main maximum region of self-mixing interference signal with variation of static length of external cavity is equal to the coherence length of laser which is less than the optical length of laser cavity. The periodical variation of the amplitude with static length of external cavity comes from self-mixing interference of multi-longitudinal mode laser,and that of the waveform is caused by nonlinear effect produced from feedback light acted on laser cavity and laser medium.