In order to calculate the valid effective area of the target through the target-missile Line of Sight (LOS) angle, a calculational method for the target-missile LOS angle was put forward. Firstly, two target-missile LOS angles were defined and described about the mean of the two angles. Then, through analyzing the impact of multi-factors to the target-missile LOS angle, a calculational method of the target-missile LOS angle was discovered by using the theory of coordinate conversion. At last, the influence of the missile’s space location, the target’s space location and the target’s three pose angels on two LOS angle was simulated. Simulation results validated the validity of the method. At the same time, the method provides some reference value for the research on the valid effective area of the target and further achieving the target tracking.