The invention discloses an automatic gas pressure regulator particularly arranged on a small-size fuel engine for use. The automatic gas pressure regulator comprises a shell, a safe gas stopping device and an automatic pressure regulating device, wherein the shell is provided with a gas inlet nozzle and a gas outlet nozzle; when the small-size fuel engine operates, vacuum generated by a piston descending in a cylinder is delivered to a negative pressure cavity inside the safe gas stopping device through an air duct to absorb a vacuum diaphragm so as to drive a rocking arm mechanism to open the air inlet nozzle, therefore the gas is regulated and stabilized by the automatic pressure regulating device in pressure, then enters the throat position of a carburetor of the small-size fuel engineand forms a air-gas mixed gas with idealer mixing ratio with air sucked by the small-size fuel engine on the throat position. The invention can enable the small-size fuel engine to be transformed into a multi-fuel purpose engine product which can not only use gasoline fuel, but also use gas fuel (natural gases, liquefied petroleum gases, methane, water gases, and the like), and ensures that the engine has little fluctuation of rotating speed and is stable in work.