In order to improve the fuel economy of light truck series hydraulic hybrid vehicle,a control guidance nonlinear mathematical model of hydraulic hybrid vehicle was proposed.In the mathematical model,a series hydraulic hybrid vehicle was regarded as a multi input and multi output system.The input was composed of the engine throttle opening,hydraulic pump displacement and maximum displacement,and the mechanical braking torque.The output was composed of engine speed,engine torque,accumulator pressure,vehicle speed and fuel flow rate.Quadratic integral liner (LQI) was designed,it was applied to the series hydraulic hybrid vehicle model,by which the control of the output of the low level controller could be realized.The simulation results show that the mathematical model and the control strategy can be effectively used to save the fuel consumption of the engine,and the control performance is good.