To reveal the effect of working oil temperature rise on hydro-viscous drive speed regulating start, by using finite element method, the modified transient Reynolds equation, energy equation and temperature-viscosity equation were solved simultaneously to simulate the startup process. The results showed that the oil film temperature rise caused by the friction pair approximately was 20℃ when inlet temperature of the working oil was 30℃, the decrease of oil film load capacity was about 9%. If inlet temperature of the working oil rised from 30℃ to 50℃, the oil film load capacity decreased by 32% even if the temperature rise caused by the friction pair was neglected. It could make oil film load capacity lower than the dead zone pressure of the proportional pressure valve used in the control system, thus the friction discs were engaged directly and could not get a perfect speed regulating start. The results were verified by experiments.