The invention discloses a pressure regulating mechanism which comprises a mechanism body and a piston assembly and a valve seat assembly which are arranged in the mechanism body. The internal space of the mechanism body is divided into an upper chamber, a middle chamber and a lower chamber through the piston assembly and the valve seat assembly; the mechanism body is provided with an atmosphere exhaust opening communicated with the upper chamber, an output opening communicated with the middle chamber and an input opening communicated with the lower chamber; pressure regulating springs are arranged above the piston assembly, the upper ends of the pressure regulating springs abut against the lower end face of a spring seat, a regulating mechanism is arranged on the spring seat, and a support spring is arranged under the valve seat assembly. According to the pressure regulating mechanism, through cooperation of the piston assembly and the valve seat assembly, the pressure of high-pressure gas entering from the input opening can be reduced to a set value, and then the high-pressure gas is output from the output opening; when the pressure of the output opening is higher than the set value, compressed gas of the output opening can be exhausted from an inlet introducing atmosphere to reduce the pressure of the output opening to the set value, and therefore the purpose of keeping the pressure of the output opening of the mechanism at the set value is achieved.