By taking a friction disk of high energy density wet clutch as research object, the thermal load on the friction disk’s involute spline was studied. Based on ANSYS mult physics fields, a three dimensional transient thermal structural coupling model was established in friction disk engagement process. By considering the impacts of composite oil grooves in spiral radial direction, the transient thermal stress and deformation of the internal spline were analyzed. The results show that the large temperature gradient is occurred in internal spline tooth portion in the engagement process, and thermal stress is significantly concentrated at the root of the tooth, which affect the engagement reliability of spline greatly under complicated working conditions. The thermal deformation in the tooth portion is varied greatly along with different positions in oil grooves, which causes the uneven load in tooth portion and the tooth fracture failure under impacting load.