This paper provides a theoretic basis to analyze the mechanism of control rod dropping and a solution method to solve the process of dropping. The authors analyze the effect of different forces acting on the control rod in detail. A mathematical model is built and a program is written to solve the process of downward motion. Finally, the model and relative coefficients of the model are modified according to the experimental data. The modified model can well simulate the process of dropping of the control rod, which gives a powerful proof to the accuracy of the model.