The invention discloses an automobile back door structure and a back door opening method, an automobile back door assembly is composed of an upper back door assembly and a lower back door assembly, and the upper back door assembly and the lower back door assembly are separately arranged, mutually independent and respectively arranged at mounting positions of an automobile body; the upper back door assembly is obliquely and upwards arranged on the upper portion of the vehicle trunk and forms an upper cover plate of the vehicle trunk after being assembled, and the lower back door assembly is vertically arranged on the rear side of the vehicle trunk and forms a rear baffle of the vehicle trunk after being assembled; the upper back door assembly mainly comprises a back door inner plate reinforcing ring assembly, a back door operation guide rail, a back door operation driving shaft, a driving motor, a connecting support, a cover plate and a back door. The upper back door assembly and the lower back door assembly respectively form an upper cover plate and a rear baffle plate of the trunk, and are respectively opened in a sliding or rotating manner under the action of electric control, so that a relatively large external space is not occupied, a rear space required for opening the back door is reduced, and the back door can still be normally opened under the condition that an automobile is parked in a narrow rear space, and luggage is convenient to take and place.