Light screen is a photoelectric test equipment that adopts the principle of measuring the flying time in a fixed distance, commonly used to measure the flying velocity of projectile which also known as the initial velocity of projectile near the muzzle. For the same diameter projectile, distance measurement error is the main factor affecting the velocity measurement error when timing error is determined. Direct measurement methods such as steel rules and laser range finders are used to measure long target distance greater than 1 m in light screen, and measurement error of distance is generally 0.5 mm, the relative error of distance is not more than 0.5‰, which has little impact on the relation error of velocity measurement. For light screen of short target distance with target distance less than 200 mm, the above method is still used to measure distance, and the distance relative error is greater than 2.5‰, then the relative error of velocity measurement exceeds 2.5‰, which does not meet the requirements of the actual velocity measurement. In this paper, a target distance calibration method using the dual zone-block velocity measuring device is proposed, which consists of two sets of calibration light screens with the same principle. The target distances of the two sets of light-screen are consistent and much larger than the distances of calibrated light screens. Based on the transmission formula of velocity measurement error, a calculation method of minimum calibration distance under the determination of that error is given. Two sets of light screen are utilized for calibration to measure the velocity of the flying projectile at two points on the ballistic separated by a fixed distance. Combined with the distance between two points, the resistance parameters of projectile and instantaneous velocity at any time between two points are developed based on the ballistic equation. Based on the fact that the average velocity within a certain distance is equal to the instantaneous velocity at the midpoint of projectile flying time within that distance, considering the time sequence of projectile passing through calibration light screen and the calibrated light screen, the distance calibration formula of calibrated light screen is given, and the calibration error calculation formula is derived. The influence of calibration distance and layout spacing in dual zone-block device on the calibration error is analyzed when projectile's flying velocity is determined. If projectile velocity is constant, the calibration error decreases with the increase of calibration distance and layout spacing. Furthermore, when the calibration distance and layout spacing are fixed, the larger the projectile flying velocity, the greater the calibration error. Taking the target calibration test of three light screen as an example, the single zone-block calibration test and the dual zone-block calibration test are carried out, in which the calibration distance is 1 237 mm and the layout spacing is 1 996 mm. When the projectile velocity is 150 m/s, the calibration relation error of single zone-block device and dual zone-block device are 1.6‰ and 0.8‰, respectively. The velocity measurement is carried out with calibration results of dual zone-block, and the relation error of velocity measurement is 0.37‰, which meets the requirements of velocity measurement. We proposes a calibration method for dual zone-block screen velocity measuring device, which indirectly measures the target distance of light screen with unknown distance by using the calibration light screen with known long target distance. Meanwhile, the calibration method proposed can be used to calibrate the target distance of sensors for zone-block velocity measurement on various principles and usage level.