For the phenomenon of high HC raw emission of GDI engine under partial transient conditions, a WLTC(world harmonized light vehicles test cycle) test was carried out on a light vehicle equipped with a GDI engine by using the chassis dynamometer. The transient HC emission, incylinder pressure and ECU control parameters of single cylinder were collected, and the reasons of high HC emission were studied. The results show that the warm-up of engine coolant has no significant effect on the raw HC emission, and the high HC emission segments appear in each driving speed segment of WLTC. When the engine torque changes sharply from high to low, the intake air sharply reduced, but the fuel may be not cut off in the cylinder, and in-cylinder misfire or incomplete combustion occur and results in high HC emission. In addition, when the fuel injects again after the cutoff of several continuous working cycles, the in-cylinder excess air coefficient may deviate from the optimal range, the in-cylinder combustion worsens, and then misfire or incomplete combustion occurs, and hence HC emission increases