Based on the actual structure of a wet multiplate clutch and the mechanism of generating drag torque loss,the different effects of oil on the friction plate oil groove and the influence on the drag torque loss under the difference of positive and negative rotational speed were analyzed,and the variation rule of drag torque loss under the difference of positive and negative rotational speed was obtained through simulation and experiment.Simulation and test results show that under the negative speed difference,due to the larger speed of the friction plate,the dynamic pressure generated on the side of the oil groove is also larger,and the drag torque loss is also larger than that with the positive speed difference.The influence of the clutch drag torque loss on the transmission shift process was analyzed.It is found the shift force required under negative speed difference is larger than that under positive speed difference.Therefore,there is a risk of difficult shifting under negative speed difference.