An engine control method and an engine control system that can inhibit the emission of NOx and reduce noise are provided. An engine control method in which an engine 2 is caused to perform diffusion combustion at normal times and switched from diffusion combustion to premixed combustion when an engine load is low, wherein when the engine 2 is switched from the diffusion combustion to the premixed combustion, an oxygen concentration in an intake air of the engine 2 is found and when the found oxygen concentration exceeds a threshold of NOx generation oxygen concentration that has been set in advance, switching from the diffusion combustion to a fuel injection mode of the premixed combustion is stopped, and when the found oxygen concentration then becomes equal to or less than the threshold of NOx generation oxygen concentration, switching is performed to the premixed combustion.