The pretension of the central gear floating NGW planetary gear reducer tumbler bearing is studied. The relative displacements of the inner and outer rings of the tumbler bearing are analyzed according to the light, medium and heavy pretension loads. The tumbler bearing is tested in the different pretension loads. In some operating conditions such as constant power, constant rotating speed and constant torque, etc., the transmission efficiency, vibration and temperature of the reducer are tested and analyzed and the dynamic characteristic curve is obtained. According to the test curve, the principles of choosing the pretension loads of the tumbler bearing are presented. The experiment of this paper is also of reference value for studying the pretension of the other types of reducer bearings.