Abstract: For the problems of low detection sensitivity, susceptibility to sky brightness and inability to work at night of existing sky screens, a detecting method was put forward which was an active light curtain based on laser reflection. A line laser semiconductor was adopted as the active light source in the design, and the light path detection system consisted of optical lens, slit diaphragm, optical filter and photomultiplier tube. When the flying projectile passed through the light curtain composed of the line laser semiconductor and the optoelectronic detection system, part of the light reflected from the surface of projectile body were received by the optoelectronic detection system. After the process of photovoltaic conversion by photovoltaic detectors and the processing of electrical signal generated by photovoltaic detectors using the subsequent signal processing circuit, the corresponding analog signal and pulse signal indicating the moment when the flying projectile passed through the light curtain were output. Light energy was analyzed and calculated, and finally tested by experiment. The results show that the method can be used at night and the sensitivity is up to more than 200 times of the projectile diameter.