Abstract: In order to achieve the velocity measurement for initiating explosive device, a new double-slit light screen was designed to realize low speed measurement for the part in initiating explosive device. The light screens designed are different from the traditional single-slit light screens. Two transmitters and receivers were installed into a common mechanical frame so that two screens were formed, which detected the moving objects going through their sensing area. Two identical devices designed are fixed on one mechanical frame with sliding rail. The distance between two devices can be easily adjusted by operator. The device can be used to measure the momentum of initiating explosive device. Compared to the traditional method with two pieces of copper as the sensor, this method improves the precision of measurement. The experiment with gas gun and real product proves that the double-slit light screen device is reliable and the result of measurement data is accurate.