A CFD model of natural gas engine was established, and working process from intake stroke to combustion stroke was simulated in this paper. Based on the validation of CFD model through experimental method, the combustion characteristics of lean-burn natural gas engine are studied under different ignition timings and different ignition energies. Results indicate that, the in-cylinder indicated mean effective pressure increases with the ignition timing advancing from 22°CA BTDC to 32°CA BTDC at the same load level. Meanwhile, the heat release rate is increased by 23.18J/°CA and its peak phase is advanced by 9°CA. The peak pressure is also increased by 45.95% and its phase is advanced by 4.5°CA. On the other hand, when the ignition energy decreases from 91.97mJ to 33.1mJ at the same load level, the in-cylinder indicated mean effective pressure decreases. Moreover, the heat release rate is decreased by 15.18J/°CA and its peak phase is delayed by 6.5°CA, the peak pressure is decreased by 22.46% and its phase is delayed by 4.5°CA. The advancing ignition timing and increasing ignition energy enlarge the flame surface density and accelerate the burning rate at the same crank angle and lead to higher combustion intensity, so they are effective to shorten post-combustion period, and advantageous to improve the economy and dynamic performance of natural gas engine. However, due to faster combustion, the in-cylinder temperature raises, which results in slight increase of NO emission.