The invention discloses a method and a device for detecting an assembly outline size of a main air bag of an automobile and belongs to the field of metrology. The method and the device are used for detecting the dimensional precision of the assembly outline size of the air bag and can eliminate the defects that existing methods are high in cost, high in difficulty, a lot in influencing factor, low in efficiency and the like. The detecting method comprises the steps of A, manufacturing a digital template of an assembly outline image of the qualified automobile main air bag; B, conducting digital transformation for the assembly outline image of the air bag to be detected; and C, matching a digital image of a workpiece to be detected with the digital template of a qualified workpiece to obtain a result. The detecting device comprises a working platform, a frame or a box body, an air bag positioning fixing mechanism, a camera support, a camera, a light source, an image processing module, an operation module and a working power supply, wherein the frame or the box body supports the platform, the air bag positioning fixing mechanism is located on an installation platform, the camera support is fixed on the platform or the frame, the camera and the light source are installed at the upper end of the support and located above a bearing platform, and the image processing module, the operation module and the working power supply are installed on the frame or the support or the platform. The method and the device have the advantages of being capable of detecting the outline size of the air bag of the automobile efficiently, quickly and accurately and applicable to on-line detection of industrial production lines.