In the primary reflection type laser screen velocity measuring technique, the scattering angle of the laser beam generated by the semiconductor laser source makes the thickness of the emergent optic screen inconsistent, and the residual divergence angle of the reflected optic screen generated by the primary reflective screen makes the thickness of the reflected optic screen inconsistent, the two aspects from which lead to the problem that the projectile passing through different positions of the optic screen to trigger the response time of the optic screen inconsistent. According to the geometrical optics principle, the mathematical model of the sagittal and meridional directions of the semiconductor laser was established, and the aspherical collimating lens groups with different surface figures were designed, the size of the exit spot was controlled within 1 mm and the divergence angles of the meridional and sagittal directions were 0.13 mrad and 0.46 mrad, respectively. After the exit beam was one-dimensionally expanded by the Powell lens, a fan-shaped exit light screen with a thickness of 1 mm and a uniformity of 85.7% was formed. And after the original reflection, the effective thickness of the reflected light curtain was effectively controlled to 1 mm with the slit aperture. Moreover, the Zemax software was used to simulate the projectile passing throuth the screen. When the projectile did not block the system laser screen, the detector received the original reflected light intensity of 1.54 mW; when the projectile blocked the system laser screen, the detector received the original reflected light intensity of 1.03 mW; when the projectile was close to the side edge of the exit laser screen (ie, 1 mm from the screen edge ), the light intensity received by the projectile trigger detector respectively at 100 mm, 300 mm, and 500 mm from the light source was all 1.54 mW. Obviously, there is no change in light intensity relative to the absence of the projectile blocking laser screen, which proves that the effective detectable optic screen thickness of the system is consistent and 1 mm. This result indicates that the research scheme is feasible.