The embodiment of the invention discloses an automatic exposure control method, applied to an X-ray machine. The method comprises the following steps: sending initial tube voltage and initial tube current to a high voltage generator, so that the high voltage generator emits X rays according to the initial tube voltage and the initial tube current, and exposure begins; detecting feedback voltage generated by an ionization chamber, thus obtaining first comparison voltage; acquiring reference voltage when the first comparison voltage is greater than or equal to sampling voltage; further detectingfeedback voltage generated by the ionization chamber, thus obtaining second comparison voltage; when the second comparison voltage is greater than or equal to the reference voltage, switching off thehigh voltage generator, and thus exposure stops. Thus, the reference voltage is set according to the practical exposure process, so that the setting of the reference voltage is more reasonable, the accuracy in controlling automatic exposure is guaranteed, an exposure image with ideal gray scale is obtained, the stable and reliable quality of the exposure image is guaranteed, and therefore, the situation that the exposure is insufficient or excessive existing due to setting of the reference voltage by virtue of experience in the prior art is avoided.