Time-varying velocity measurement by self-mixing laser diode is studied. A signal processing based on polynomial phase parameter estimation is presented. Based on the principle of self-mixing laser diode velocity measurement,polynomial phase expression of the laser diode output is deduced when the self-mixing laser diode is used to measure time-varying velocity. Namely,a model is proposed for the simulation of time-varying velocity measurement. At last,the signal processing based on polynomial phase is used to obtain target’s velocity and acceleration . The simulation results prove the validity of the algorithm even in the situation of low SNR.