A new structure of hydraulic hybrid vehicle and the working principles were presented. The characteristics of Hydraulic Transformer (HT) displacement and pressure were analyzed after the mathematical models of HT displacement and pressure were established. Results showed that the various parameters of the HT port plate control angle from regular pattern were obtained when the energy regenerative system was working. Moreover, according to the optimal conditions of the accumulator, an optimization energysaving control algorithm with various operation conditions was established. Finally, the control relationships between optimization energysaving algorithm and various parameters of HT and accumulator were obtained through simulation tests when the HHV operated in different working conditions. So, the optimization energysaving algorithm can use HHV to achieve maximum energy recovery.