The system that displays and controls antiaircraft gun projectile flight characteristics uses data from host computers to realize 3D ballistic trajectory simulation, then compares the results with ideal ballistic trajectories in order to judge whether the accuracy of ballistic trajectory satisfies the requirements and to evaluate whether the firepower technology achieves the expected results. This paper briefly introduces the basic framework and functions of the system and presents a cubic spline interpolation fitting algorithm for 3D ballistic trajectory simulation. The algorithm decomposes the 3D discrete characteristic data into two 2D trajectories separately in the launch plane and the horizontal plane, then interpolates and fits them by a piecewise cubic spline interpolation algorithm respectively, and finally plots the 3D ballistic trajectory according to the interpolation results. The algorithm is realized by Microsoft Visual C++ 6.0. Through an evaluation instance, the 3D ballistic trajectory is shown to be smooth and can pass through each characteristic point, meeting the fitting condition as well as the requirement of trajectory simulation.