Abstract: Based on the analysis of the rod drop process of rod control cluster assemble（RCCA）and the action of control rod drive mechanism gripper and the response of the primary coil of the rod position detector, this paper presents the automatic judgement, the analysis of rod drop wave and the automatic calculation of the rod drop time, and performs the design with the virtual instrument technology. The relevant design is validated by the real rod drop wave, and the test result shows that it can well and truly capture the rod drop wave and automatically calculate the rod drop time. The production has been applied in Hainan Changjiang Nuclear Power Plant and Fujian Fuqing NPP unit 3 and unit 4.