Automatic mechanical pressure-stabilization expansion water tank cover

The utility model provides an automatic mechanical pressure-stabilization expansion water tank cover. The automatic mechanical pressure-stabilization expansion water tank cover comprises a filler cap, wherein the filler cap is clamped on a filler outer opening of an expansion water tank in a covering mode, a constant pressure air bag, an upper plate, a spring and a lower plate are sequentially and coaxially arranged in a vertical space between the filler cap and a filler inner opening from top to bottom, the upper surface of the constant pressure air bag abuts against the inner wall of the filler cap, the upper surface of the upper plate makes contact with the lower surface of the constant pressure air bag, the upper plate is parallel with the lower plate, the spring is vertically arranged between the upper plate and the lower plate, the lower surface of the lower plate abuts against the end surface of the filler inner opening in a sealing mode, and the filler cap is provided with an air hole communicated with the inside of the filler outer opening.