The lens light-screen detection sensor with L-shaped LED light source constitutes a large area rectangular detection light screen, which is usually arranged in a closed indoor test target to measure exterior ballistic parameters. The sensitivity of the detection area directly affects the measurement accuracy. The spatial distribution of sensitivity of 10m×10m large area rectangular detection light screen with L-shaped LED light source is been studied and analyzed , and the len edge effect and light propagation path is considered .The detection area is divided into small blocks, the change of luminous flux caused by the same caliber projectile passing through different blocks is calculated by digital simulation, and the spatial distribution of sensitivity of rectangular detection light screen area is obtained. The results show that the sensitivity distribution of the rectangular detection area is symmetrical. When it is closed to the main optical axis of the lens and near the lens, a larger detection sensitivity in the detection area will be obtained. The research results provide a theoretical basis for the subsequent large target exterior ballistic testing technology.