The combustion characteristics of injected natural gas based on experiments were carried out in a constant volume vessel. The influences of equivalence ratio, temperature and premix ratio on the flame morphology and combustion characteristics were analyzed. The flame front is wrinkled and twisted during the flame propagation, which shows the extreme randomness of injection combustion caused by the strong turbulence. The maximum pressure and maximum heat release rate of injection combustion are larger than premixed combustion, with less time reaching the maximum pressure, according to the progress of the pressure in the vessel. The initial and main combustion durations of the injection combustion are also shorter than that of premixed combustion.