The reforming reaction of natural gas was conducted with the external fuel reforming system and the reforming rate was 12%. The reforming gas after reaction was introduced into the intake pipe and mixed with fresh air, and then charged into the cylinder for combustion so as to compare the influence of external fuel reforming hydrogenmixed combustion on the performance of natural gas engine. The experimental results show that the cycle variation of engine significantly improves, the combustion duration shortens, and the fuel consumption rate under the same working conditions changes little after the application of external fuel reforming hydrogenmixed combustion. THC emissions significantly reduce under different working conditions, and the relative reduction reaches 22% under 50% torque load rate. In addition, NO_x emissions reduce and the relative reduction reaches 15% especially under low loads. However, the CO emissions still has a rise to different degrees because the reforming gas itself contains a certain amount of CO, coupled with hydrogen reduction.