In order to improve character of combustion of CNG engine, basing on type 6105 turbine charge CNG engine, the air flow in the cylinder and its combustion characteristic affected by different combustion chamber’s shape were simulated numerically. The result shows that the shape of combustion chamber plays an important role in the squish formation and combustion process. The contracted cave type combustion shape (chamber C) had much stronger squish and longer term period lasting compared with the others. The flame propagation velocity could be increased and the best combustion performance could be got. But the thermal load near igniter plug and the mass fraction of NOx produced during combustion in chamber C was higher. On the contrary, open cave type combustion (chamber B) had much weaker squish and slower flame propagation velocity. The worst combustion performance would be got. The situation of air flow in the cylinder and its combustion performance of vertical cave type combustion shape (chamber A) was between chamber C’s and chamber B’s. The suitable flame propagation velocity and good combustion performance could be got. And the thermal load near igniter plug and the mass fraction of NOx produced during combustion could be reduced. It was a suitable combustion chamber for CNG engine.