Wet clutches, as essential components of powertrain systems, are widely used in the transmissions of energy-efficient and new-energy vehicles. However, during operation, wet clutches generate drag torque due to the shear effect of the cooling lubricant oil film when the clutch plates are disengaged. This drag torque reduces transmission efficiency and affects the system’s dynamic characteristics. Therefore, this thesis initially experimentally investigates the influence of different factors on the drag torque of wet clutches. Subsequently, the thesis experimentally examines how drag torque varies with the service mileage of the wet clutch and how the duration of lubricant use affects drag torque. By analyzing the three-dimensional profiles of friction plate surfaces with varying service mileage, this study reveals the mechanism behind changes in drag torque as wet clutches accumulate mileage. Finally, a theoretical model was developed to describe how drag torque changes with service mileage, considering the effects of service mileage on the lubricant coverage area between friction surfaces.