Aiming at the problem of smoke under transient conditions, the control strategy of smoke limit based on torque coordination was designed to realize the smoke emission control of diesel engine under transient conditions. The effects of different excess air coefficient limits on the power performance and emission of diesel engine under transient conditions were systematically studied. The results show that the excess air coefficient limit can limit the cycle fuel injection of diesel engine during the transient process so that the expected excess air coefficient was acquired and the smoke emission is hence controlled. The smoke emission is less and the transient power response is slower at the excess air coefficient of 1.51. Compared with 1.01 excess air coefficient, the peak smoke decreases by 85% and 88% and the transient dynamic response delays by 58% and 57% at 1 200 r/min and 3 200 r/min respectively. In order to meet the power requirement and prevent the higher smoke emission due to a large excessive air coefficient under transient conditions, an appropriate excess air coefficient limit should be configured.