Trajectory simulation is an important tool for improving the accuracy of missile. Up to now, most previous works have simulated only the ideal two-dimensional trajectory and the results are not practical. This paper tries to simulate the trajectory curve of the missile in three-dimension by decomposing it into two two-dimensional planes and guiding the missile in the two planes respectively with proportional guidance method. Considering the influence of target’s maneuverability on the trajectory, the curves of decomposed motion and composed motion are plotted and they show how the moving parameters of the missile and target change. The influences of different missile velocities and guidance constants on the trajectory are compared and a general relationship between the guidance constant and normal over-loading is derived, which may be advisory in practice. The simulations are realized on MATLAB.