The accuracy of the flying projectile speed measurement is directly affected by the detection light screen sensitivity of the split large-target test system. In the case of small dispersion, the artificial light source in the rectangular large-area test system can be replaced by a reflective film and a laser to simplify the test system. The sensitivity of the new triangular detection light screen was analyzed. The light intensity attenuation of lasers at different propagation distances, the non-uniform distribution in space, the retro-reflection coefficient of the reflective film and the off-axis effect of the lens were taken into account. The sensitivity of projectiles of the same diameter through different positions of the light screen was analyzed by equivalent normalization at the same reference point by using numerical simulation and firing test. The simulation results show that the sensitivity is most affected by the lens off-axis effect and least by the non-uniform distribution of the laser space. The firing test was carried out in the right-angled triangle detection light screen area of 3.5 m×2 m (width×height). The results are consistent with the simulation results, the closer the lens, the higher the sensitivity, and the lower the versa. The results can provide a reference for the engineering design of a large-area detection light screen similar to the reflection type.