Abstract: The key parameters influencing cold start HC and NOx emissions, including motoring speed, “fuelon” speed and ignition retard, were analyzed by studying engine start ignition, fuel delivery and air charging strategies. The results show that higher motoring speed helps to reduce start fuel, improves fuel atomization and completes combustion, and thus reducing the HC emission. But too high a motoring speed will deteriorate NOx emission. Ignition retard can accelerate TWC lightoff effectively, but too much of it will cause unstable combustion and misfire, and deteriorate HC and NOx emissions simultaneously. By studying the law of start emission, the optimized parameters were selected, laying a foundation for further accurate emission calibration.