Two cam phases were designed basing on original diesel engine which was refitted into spark ignition CNG engine. The influence of cam phase on combustion and exhaust was studied by experiment on indoor test. The conclusions are showed as follows. With the smaller valve overlap, the exhaust of HC and NOx reduces; but too small valve overlap at high speed is adverse to the exhaust of HC. The density of burning mixture reduces with smaller overlap and the growth of NOx is restricted.On the other hand，power of the engine also reduces. So, both the power and the exhaust should be considered at the same time. The crank angle of ignition advance should increase properly at high speed,in order to keep the power and avoid the post combustion and reduce the temperature of exhaust. 