Recently, as vehicles are getting more intelligent, interest in improving vehicle ride comfort has been increasing. Appropriate control of a clutch in the vehicle powertrain can play a major role in improving the vehicle ride comfort. Previously, many researches on tracking controllers of clutch slip speed and clutch transmitted torque in clutch slip engagement control have been conducted. However, little research on an optimization method of the reference clutch slip speed and clutch torque has been performed. Therefore, in this study, the optimization method of the reference clutch slip speed in clutch slip engagement in the vehicle powertrain is proposed to improve clutch lifespan and vehicle ride comfort. The proposed method is verified through clutch slip engagement simulations using AMESIM and MATLAB Simulink.