The study on combustion characteristics of different excess air ratios (λ) and ignition timings was conducted on a lean-burn spark-ignition CNG engine, and the corresponding emission characteristics was also analyzed. The results show that the curve of cylinder pressure moves towards TDC, and the CoVimep decreases with the advance of ignition timing. With the excess air ratio increasing, the CoVimep increases while the cylinder pressure decreases firstly, and then increases. The flame development duration increases with the ignition timing advanced, but the rapid flame duration decreases, the total combustion duration basically keep constant, and all of them increase with the increasement of excess air ratio. Best fuel economy was gotten at λ=1.2. With the excess air ratio increasing, the CO and NMHC emissions decrease, the NOx emission firstly reaches to a peak value, which is about 3000ppm at λ=1.2, and then decreases. Both of the THC and CH4 emissions have opposite trends to NOx emission. All the emissions mentioned above increases with the advance of ignition timing.