Reduction of drag torque is one of important potentials to improve transmission efficiency. Existing mathematical model of drag torque was not accurate to predict the decrease after oil film shrinking because of the difficulty in modeling the flow pattern between two plates. Flow pattern was considered as laminar flow and full oil film in the gap between two plates in traditional model.Subsequent equivalent circumferential degree model presented an improvement in oil film shrinking due to centrifugal force, but was also based on full oil film in the gap, which resulted difference between model prediction and experimental data. The objective of this paper is to develop an accurate mathematical model for the above problem by using experimental verification. An experimental apparatus was set up to test drag torque of disengaged wet clutch consisting of single friction and separate plate. A high speed camera was used to record the flow pattern through transparent quartz disk plate. The visualization of flow pattern in the clearance was investigated to evaluate the characteristics of oil film shrinking. Visual test results reveal that the oil film begins to shrink from outer radius to inner radius at the stationary plate and only flows along the rotating plate after shrinking. Meanwhile, drag torque decreases sharply due to little contact area between the stationary plate and the oil. A three-dimensional Navier-Stokes (N-S) equation based on laminar flow is presented to model the drag torque. Pressure distributions in radial and circumferential directions as well as speed distributions are deduced. The model analysis reveals that the acceleration of flow in radial direction caused by centrifugal force is the key reason for the shrinking at the constant feeding flow rate. An approach to descript flow pattern was presented on the basis of visual observation. The drag torque predicted by the model agrees well with test data for non-grooved wet clutch. The proposed model enhances the precision for predicting drag torque, and lays down a framework on which some subsequent models are developed.