In an engine control method for maintaining performance of oxidation catalyst, fuel of a compressed natural gas engine under a lean-burn condition with an air-fuel equivalence ratio less than or equal to 1 is burned. Activation of the oxidation catalyst for purifying an exhaust gas of the engine is determined. Necessity of a regeneration mode for recovering catalytic activation of the oxidation catalyst according to an activation state of the oxidation catalyst and an operating condition of the engine is determined. And, the air-fuel equivalence ratio is increased to a range of from 1.10 to 1.20 when the oxidation catalyst is inactive and an engine speed is lower than a predetermined speed.