A metering pump with a special-shaped cavity comprising a housing, a rotor and two cover plates. The housing is a cylinder with a special-shaped surface inner cavity formed by combining two circular arc surfaces with two non-circular arc surfaces and a planar surface, an inlet and an outlet. The rotor comprises a rotor body and two pairs of combined sliding plates. The housing is matched with the cover plates for constituting a sealed cavity. When the rotor rotates, the non-circular arc surface of the special-shaped surface inner cavity can enable the two pairs of the combined sliding plates to slide in a cross manner so as to suck in fluid from the inlet and press out the fluid from the outlet. When the rotor rotates one cycle, four standard volumes are formed in a cavity body and the equal quantity of the fluid flows by the cavity body.