A modified moment acquisition method of projectile going through a trapezoidal screen in certain angle was put forward,for mid-screen trigger has not taken measuring error caused by the screen′s uneven thickness distribution into consideration in tradition.Firstly,the ratio of the position for the central plane and the distance for the projectile flight in the screen was calculated,then converted the ratio to the moment for the projectile trail arriving at the screen thickness′ central plane and the time for the projectile going through the whole screen.Furthermore,using the iterative algorithm extracts the moment of the projectile trail arriving at the central plane,thus the space-time parameters of the projectile was calculated at the same time.The method was verified by the real projectile tests,and the results show that it has the capability to reduce 1mm measuring error of the six-light-screen array sky screen in certain range.