In order to study the engagement characteristics of wet clutch, the mathematical model of wet clutch engagement process was established based on fluid lubrication theory, rough surface elastic contact theory, adsorption heat theory and heat transfer theory, with the combined effects of friction surface temperature, relative speed, roughness and load on the friction coefficient taken into consideration. The effects of engagement pressure, friction surface roughness, friction material permeability on oil film thickness, relative angular velocity and transfer torque were discussed respectively. The results show that with the increase of the engagement pressure, the torque response, the relative angular velocity reduction speed and the oil film thickness reduction speed are accelerated, but the engagement time is shortened and the minimum oil film thickness is reduced. With the reduction of the friction surface roughness, the torque response slows down, the relative angular velocity reduction speed and the oil film thickness reduction speed are accelerated, the engagement time is shortened, and the minimum oil film thickness is reduced. With the increase of the friction material permeability, the torque response, the relative angular velocity reduction speed and the oil film thickness reduction speed are accelerated, and again the engagement time is shortened, but the minimum oil film thickness barely changes.