Abstract: The behavior of engine automatic start and stop under idle speed is an important working mode for a mild hybrid electric vehicle (HEV), which can avoid engine running under idle operating mode. So it can reduce engine fuel consumption and exhaust gas emission. The engine start dynamics and the control strategy of start/stop system for ISG(integrated starter/generator) mild HEV were studied. The hybrid controller unit (HCU) model was built in Matlab/Simulink environment, and the coordinated output torque between engine and ISG motor was simulated in the NEDC driving cycle. Finally the engine start and the drum contrasted test were carried out. The simulation and test results showed that the start/stop system could ensure the engine stop normally under idle operating mode. With the start/stop system coordinated control, the engine run smoothly in the whole driving cycle, and the output torque was always in the low fuel consumption area. The HC and CO emissions of HEV equipped with start/stop system declined obviously compared to the traditional vehicle. The 100-kilometer fuel consumption dropped from 8.60 liter to 7.30 liter, and the fuel saving effect was remarkable.