The invention belongs to the technical field of transport operation platforms and specifically relates to a top-entry-type large-span platform automatic support mechanism comprising a transport operation platform and support assemblies arranged on two sides of the transport operation platform in pairs oppositely. Each support assembly is a planar hinge joint connecting rod structure comprising a hydraulic cylinder, an upper support arm and a lower support arm which are hinged successively, wherein one corner end of the upper support arm and the cylinder wall of the hydraulic cylinder are hinged to the upper platform top of the transport operation platform, a piston rod end of the hydraulic cylinder is hinged to one side wall of the upper support arm, the lower support arm is in an overhung arm body structure, and hinge joint shafts at the hinge joint portions are parallel to each other. The automatic support mechanism also comprises follow-up locking assemblies. The top-entry-type large-span platform automatic support mechanism can effectively maintain the original maneuverability indexes such as the ground clearance and departure angle of the transport operation platform, effectively guarantee the transport maneuverability and safety of machine bodies, and also improve the transverse support stability.