Traditional platform seeker can directly obtain the LOS rate by its physical tracking loop, while the strapdown seeker cannot. Losing the ability to directly measure the LOS rate, the strapdown seeker need digital calculation methods to obtain the LOS rate. To solve the problem, the LOS rate estimation model was established. Considering the strong nonlinearity of the estimation model, the strong track cubature Kalman filter(STCKF) was used for the LOS rate estimation. Based on the typical launch conditions of small air-to-surface missile, the trajectory simulation was done to verify the validity of the estimation algorithm. Simulation results showed that, after capturing the target during terminal phase, the seeker can quickly eliminate filter initial error and accurately track the real LOS angles and LOS rates. To verify the feasibility of the filter output as guidance information, Monte Carlo method was used under typical random disturbances of missiles with semi-active laser strapdown seeker. The results of 1 000 shooting show that the guidance precision to stationary target is 0.58 m(CEP), and to move target is 0.84 m(CEP), which meet the accuracy requirements of precision guided weapons.