In view of the traditional measuring method based on the triangle principle strict requirements on the structure parameters and difficult alignment, a new measurement model based on the space geometry principle was put forward. The corresponding plane equation of the two incline light screens calibrated was carried out by the least square method. The projectile flying velocity and the impacting coordinates would be calculated by solving the simultaneous equations among the ballistic trajectory line equation and the four light screens’ equations. The measurement method reduced the demands and simplified difficulty of the installation and adjustment. The method was verified by the real fire tests. The results show that the planar equation model and algorithm can be used successfully. Above all, the model and algorithm simplified the installation and adjustment of the four light screen array structure saved manpower and improved the measurement accuracy.