Some embodiments of the present disclosure provide a method for judging four key moments in a whole process (a feeding starting moment, an idling process, a cutting starting moment, cutting, a cutting ending moment, an idling process and a feeding ending moment) of a machining step for computer numerical control machine tools. The method includes that: the feeding starting moment and ending moment are determined according to a real-time execution state of a numerical control code; input power of a spindle of a machine tool is compared in real time with idling power to determine the cutting starting moment and ending moment; and for a special machining step in which power does not change obviously in a feeding process, the cutting starting and ending moments are determined in combination with a historical statistical average time of an idling process duration and a feeding moment.