Abstract: The control drum system is important safety level device for power regulation and emergency shutdown of the space nuclear power device. The safety of the reactor directly depends on whether the control drum system can operate well or not. In order to test the scram time of the control drum system, a method of judging the zero position and calculating the scram time was proposed by analyzing the rotation and transmission process of the control drum system. The full-scale model of the control drum system experimental prototype and a comprehensive test platform were used to test the scram time. The test results show that the test method is real, effective and reliable, and can be applied to the measurement of scram time in the development and use of control drum system at all stages. At the same time, it is also verified that the design of the control drum system meets the design target that the mechanical scram time is less than 1 s.