The invention provides a pretightening tool and a pretightening method of a joint bearing for a pressure container checking manipulator of a nuclear power plant. The pretightening tool comprises a support frame for supporting and positioning the joint bearing, and a telescopic oil cylinder inserted in the support frame and connected with a hydraulic pump; the hydraulic pump is provided with a pressure gauge, a pretightening hammer connected with a moving end of the telescopic oil cylinder and a pretightening pressure plate connected with a rotating box cover; the pretightening hammer is inserted in the joint bearing during pretightening; the pretightening pressure plate is inserted in the top of the pretightening hammer; the pretightening hammer is provided with a radial projection capable of ejecting the pretightening pressure plate; and the pretightening pressure plate is linked with an angular contact ball bearing on the lower side through the rotating box cover. The pretightening tool uses a mode of combining hydraulic driving force with staff manual operation for pretightening, is easy to operate, and improves the assembly precision.