In order to improve the combustion efficiency of dieselnatural gas dual fuel engine at medium and low loads and reduce carbon monoxide (CO) and unburnt hydrocarbon (UHC), the CFD model for combustion chamber was built and the numerical simulation was conducted by using the method of 3D-CFD coupled with diesel-natural gas-hydrogen (DNH) three fuel chemical kinetic mechanism. The influences of intake mixed with hydrogen on combustion and emission characteristics of dual fuel engine were studied. The results show that the in-cylinder key free radical generation improves, the combustion rate of natural gas (NG) accelerates, and the combustion efficiency can reach 95.7%, which increases by 11.3% after adding 20% H₂ into the intake. The addition of 25% H₂ can reduce the total hydrocarbons (THC), CO and CO₂ by 49.0%, 37.4% and 12.7% respectively.