Using the ISG type mild hybrid electric vehicle(HEV) as research object, a Hardware-In-Loop simulation system of hybrid electric powertrain had been built by using dSPACE. A Hardware-In-Loop simulation experiment of hybrid electric powertrain control unit was carried out based on CAN, and the dynamic characteristics of hybrid electric powertrain was tested and analyzed under different working modes such as engine start-up, accelerating assist and battery charging at equable speed. The research lays a foundation for control strategy optimization of hybrid electric powertrain control unit.