Abstract: The reactor control rod drive line is an important part for the reactor safety control function. It consists of the control rod drive mechanism, the control rod assembly and its guiding system, which is the only structural equipment with relative movement in the heap. Under the ocean conditions, the working environment of the drive line of the floating nuclear power plant is more complicated for the influence of wind and sea water flow. Dropping time is an important indicator of the drive line design assessment, and the main factors that affect the time of rod dropping include: gravity, buoyancy, fluid resistance, mechanical friction and so on. In this paper, the method of theoretical simulation and software simulation is used to obtain the simulation method of control rod drive line under ocean conditions. The comparison between the simulation results and the experimental data verifies the rationality of the obtained method.