The CFD model of 2135G natural gas engine was established, and working process from intake to combustion was simulated in this paper. Based on the validation of CFD model through experimental method, the combustion characteristic of natural gas engine was studied under different ignition timings and different spark energies. Results indicate that, the in-cylinder indicated mean effective pressure was increased with the ignition timing advancing from 22°CA BTDC to 32°CA BTDC in the same load. Meanwhile, the heat release rate was increased 23.18J/°CA and its phase was advanced 9°CA, the peak pressure was increased 45.95% and its phase was advanced 4.5°CA. On the other hand, when the spark energy decreases from 91.97mJ to 33.1mJ in the same load, the in-cylinder indicated mean effective pressure was decreased. Moreover, the heat release rate was decreased 15.18J/°CA and its phase was delayed 6.5°CA, the peak pressure was decreased 22.46% and its phase was delayed 4.5°CA. When the ignition timing was advanced from 22°CA BTDC to 32°CA BTDC and the spark energy was increased from 33.1mJ to 91.97mJ, the flame surface density was enlarged and the burning rate was accelerated at the same crank angle, meanwhile, higher combustion intensity was generated, so they are able to effectively shorten post-combustion, and advantageous to improve the economy and dynamic performance of natural gas engine. However, due to faster burning rate, the in-cylinder temperature was raised, which result in slightly increase of NO emission.