Abstract: When the split detection light screen is used to test projectile velocity of the indoor target trajectory, the sensitivity of detection light screen directly affects the accuracy of initial velocity measurement. The sensitivity distribution of a large-area triangular detection light screen formed by a line light source coupled with a lens-type receiving device was studied. Considering the off-axis effect of optical lens and the attenuation of line light source with the distance, the illuminance at different positions in the detection light screen was equivalent to the line light source. Assuming that the thickness of the light screen was uniform and the projectile length was always greater than the thickness of the screen, the ratio of the area formed by the projectile blocking the detection light screen to the cross-sectional area of the light screen in the current area was equivalent to the ratio of the projectile diameter to the width of the light screen at the current position. The theoretical simulation and live ammunition test verification were carried out on the 4.8 m×2.4 m triangular detection light screen. The simulation analysis results are consistent with the live ammunition test results. The sensitivity is higher near the lens and lower far away from the lens, and the standard deviation of the analog voltage amplitude in the key detection area was 0.05 V, from which the uniformity meets the test requirements. The research results can provide references for the engineering design of the triangular detection light screen.