The application, which relates to a numerical control system, discloses an online detection and optimization method and system for a spindle resonance rotating speed, thereby guaranteeing the timeliness and detecting the resonance rotating speed of a spindle of a numerically-controlled machine tool simply, conveniently, and practically. The method comprises: controlling a numerical control systemto detect a G code and thus enabling a spindle of the numerical control system to make spinning at different rotating speeds; during the G code detection process, acquiring a vibration signal in realtime from a vibration sensor disposed at the spindle and acquiring a currently performed G code row number synchronously from the numerical control system to obtain a corresponding relationship between the G code row number and the vibration signal; and calculating the resonance rotating speed, of the spindle based on the corresponding relationship between the G code row number and the vibration signal.