In order to online quickly check the composition’s weight in bullet jacket, firstly, used reflecting fiber sensor to measure the height of the composition in bullet jacket. Because the weight of the composition is proportional to the height, so measure the weight of the composition in bullet jacket by the reflecting fiber sensor. Using this principle, designed a set of measuring system for checking the composition’s weight, and then, tested this system. And the result proved that this reflecting fiber sensor measuring system can measure the composition’s weight exactly, and the measurement precision could reach to 0.1g .