Abstract: In the process of rod cluster control assemblies（RCCA） dropping in PWRs, the guide thimble of the fuel assembly supplies a tunnel for the control rod, and the dashpot structure which is designed below the guide thimble, in order to decrease the impact force and ensure the safety of the fuel assembly. In this paper, with the CIGAL and SAM program, the analysis about the RCCA dropping and damping process is carried out to obtain the drop time and the impact force on the top nozzle in the series CF fuel assembly which includes CF2 and CF3. The calculation results demonstrate that: from CF2 to CF3, with the increasing of the screw hole length and the decreasing of the guide thimble inner diameter, the drop time gets longer, the max impact force on the top nozzle gets smaller, the damping time in the dashpot gets longer, and the pressure in the dashpot get larger.