Single-lens sky-screen is commonly used as indoor ballistic range velocity measurement device with linear array light source constitutes an isosceles triangle detection screen. The detection sensitivity directly affects the velocity measuring accuracy of sky-screen. The construction and measurement principle of detection screen are introduced, the sensitivity calculation model is set up, the calculation formula of luminous flux variation caused by the projectile shielding light source when projectile passing through detection screen is induced based on the edge effect of the optical lens and the principle of illumination attenuation with distance; The sensitivity distribution within the detection screen is obtained and verified by experimentation, normalized luminous flux variation produced by projectile passing through the optical axis as reference value. The conclusions provide guidance for the use and design of the velocity measurement system of the sky-screen with light source.