The utility model relates to a pressure-boost type fuel tank cap, which comprises an air cylinder, a fuel tank cap arranged at the top part of the air cylinder and an inflating connecting rod arranged in the air cylinder and extending out of the fuel tank cap, wherein an inflating pressure plate is arranged at the top end of the inflating connecting rod, and a piston is arranged at the tail end of the inflating connecting rod; and an air outlet hole is arranged at the tail end of the air cylinder, and a rubber ring playing the function of a non-return valve and covering a breathing hole is sleeved at the tail end of the air cylinder. A cover plate and an air valve shifter plate in axial transmission connection with each other are arranged between the fuel tank cap and the inflating pressure plate in sequence, and a spring is arranged between the cover plate and the air valve shifter plate; a breathing hole suitable for being opened and closed by rotating the air valve shifter plate penetrates on the cover plate and the fuel tank cap at the outer edge of the air cylinder, that is, the breathing hole can be closed when the fuel tank is pressurized through the piston, thereby ensuring the pressurizing effect; and the breathing hole can be opened after the generator is started up, so as to avoid the phenomenon that the negative pressure is caused in the fuel tank since no air enters, and the generator can not be operated.