Abstract: According to the motion and force characteristics of the control rod bank during rod dropping, and the test results in nine nuclear power plants, the power of fluid inside the primary loop has no impact on the rod drop time, which decreases with the increase of the temperature of the fluid inside the primary loop. The new criterion of the rod drop time is calculated in different test conditions. The conclusion cannot be only applied to the other nuclear power plants of similar type during commissioning and normal operation, but also as reference when analyzing the dynamic force model during rod dropping.