CNG-diesel dual fuel combustion mode has been regarded as a practical engine operation strategy because it not only can remain the high thermal efficiency of diesel engine but also can make full use of an alternative fuel—natural gas，which has excellent advantages in economic performance and environmental protection. However, it is suffering from misfire and high HC emissions problems under cold start and low load conditions due to the low combustion speed and high ignition energy of natural gas. As known, hydrogen is a clear energy which has extremely high flammability. Thus, a certain proportion of hydrogen can be added in the natural gas (named HCNG after blending) to improve combustion. In current work, the effect of hydrogen addition on the combustion and flame propagation characteristics of dual fuel engine was studied based on an optical engine by using high speed camera to capture the flame images and Ki-Box to collect the cylinder pressures. The hydrogen volume fraction in HCNG was varied from 0% to 30%, while the energy substitution rate of HCNG was remained at 70%. The results show that, at the 70% energy substitution rate, with the hydrogen volume fractions increasing, the ignition delay period and the combustion duration decrease，and heat release rate reaches its peak earlier, while the maximum combustion pressure and the maximum pressure rise rate are increasing. It can be shown that adding a certain proportion of hydrogen in natural gas can speed up the burning speed and improve the combustion characteristics of the CNG-diesel dual fuel engine.