In order to deeply study the NO_x formation mechanism of hydrogen fueled internal combustion engine (HICE), a hydrogen fueled internal combustion engine CFD simulation model including three-dimensional gridding coupling detailed chemical reaction mechanism is built based on CONVERGE software, and the combustion and emission characteristics of hydrogen fueled internal combustion engine under different loads are researched. The simulation result is consistent with the experimental data. The simulation results show that the increasing concentration of hydrogen is beneficial to improving the efficiency of hydrogen fuel internal combustion engine. The large amount of NO generates in the rapid combustion period, the total amount of NO decreases constantly after the rapid combustion period, and the total amount of NO tends to be stable when the average temperature is below 2 200 K. NO is generated mainly through three paths including thermo NO, NNH and N₂O, and the thermo NO path contributes a large proportion of total NO emissions, whose contribution ratio increases with the increase of fuel-air equivalence ratio. NNH and N₂O contribute about 25% of total NO emissions at lower concentrations, while when fuel-air equivalence ratio is 1.0, the sum of the contributions of these two paths to NO generation is negative. The contribution ratios for NO generation of three paths under different loads are obtained using chemical reaction kinetics method, which can reveal the NO_x generation mechanism of hydrogen fuel internal combustion engine and establish the theoretical foundation for subsequent study.