To improve the transient response of exhaust turbocharging engine under accelerated conditions, a method of steam-assisted turbocharging was proposed. Based on an exhaust turbocharged gasoline engine, the simulation models of steam-assisted turbocharging and exhaust turbocharging engine were established and then calibrated by the software of GT-Power. On this basis, the effect of various steam parameters on the performances of turbocharging system and the gasoline engine was studied at the engine speed of 2000 r/min. The results show that, at the accelerated condition of 2000 r/min, the retarded time of the gasoline engine can be reduced by 48.0% and the peak torque can be increased by 9.7% through steam-assisted turbocharging. As a result, the acceleration and dynamic performance of the gasoline engine can be effectively improved by steam-assisted turbocharging.