Aiming at the problem that inertial measurement unit can not meet the operation requirement of the low cost guidance munition, a line-of-sight (LOS) angle estimation method of strapdown semi-active laser-guided munition based on ideal trajectory was proposed. According to the relative motion model between ammunition and target and the seeker measurement model, the nonlinear filter system was built. Aiming at the features that the ballistic parameters varied in the small range, this algorithm took the ideal trajectory parameters as filter parameters by analyzing the impact from projectile motion parameters on the system uncertainty; the LOS angel was estimated by using the measurement information from laser semiactive seeker and the cubature Kalman filter (CKF). The digital simulation results showed that under the condition of small disturbance, the root mean square errors (RMSE) of LOS inclination and declination are 0.182°and 1.668°, the max estimation errors of them are 0.259° and 2.913°. The algorithm has better estimation precision and robust property.