The invention relates to a digital X-ray machine automatic exposure control method and device, and belongs to the technical field of digital radiography. The method mainly includes the steps of processing a pre-exposure image in secondary exposure and optimizing exposure parameters in a self-adaptive mode. The optimal threshold value is correspondingly found according to positions through gray stretch of the pre-exposure image, the image is divided into different areas, areas of interest with different components are extracted, the average gray level, the signal to noise ratio and the contrast ratio are calculated respectively, and then the main exposure parameters (mA, s and KV) are optimized in the self-adaptive mode; the precise time sequence is provided for the exposure process and data acquisition through a synchronous control unit, and the imaging effect is prevented from being influenced by time delay or antedating reaction. By the adoption of the multiple components, multiple target areas and multiple image standards, the exposure parameters are adjusted through a self-adaptive algorithm, and the top-quality image can be obtained through a small exposure dose when exposure is conducted under the synchronous control.