Background: The drop time of control rod assembly is one of the most important parameters to ensure the safe operation of nuclear power plants.Due to the fluid-structure interaction（FSI）

the elastic structures

such as control rods and guide tubes

will vibrate in the dropping of control rod assembly.The impact and friction between the control rod and guide tubes caused by large transverse vibration will influence the drop time calculation.Purpose: To study in detail the flow-induced vibration and the friction

this paper focus on the fluid-structure interaction in the control rod drop process.Methods: Firstly

the vibration equations of control rod and guide tubes considering the fluid-structure interaction are established.Then the various fluid forces are analyzed in accordance with their qualities

and the influences of different guide tubes in a guide tubes array are also considered.Results: The friction between control rod and guide tube is not zero

and the friction under seismic condition is larger.Conclusions: The analysis on the fluid and structure interaction presented in this paper is reasonable and can improve current analytical models of control rod drop time calculation.