The utility model discloses a mud pump piston seal ring. A front lip with a cusp-shaped cross-section is arranged in the front end of the seal ring body, one or two rear lips with a cusp-shaped cross-section are installed in the middle of the seal ring body, the cusp of the rear lip has the same direction as that of the front lip, and the outer edge of the rear lip is 0.1 to 0.3 millimeters higher than that of the front lip. The intersection angle formed by the rear lip and the seal ring body at the joint position is set to be a transitional fillet. The mud pump piston seal ring has a compact and reasonable structure possessing the function of two seal rings. The front lip is mainly used to push off large particles of impurities from lubricating medium, and the rear lip remains small particles of impurities which can not be pushed off by the front lip into the intersection angle formed by the rear lip and the seal ring body to reduce the frictional resistance. Accordingly, the seal ring surface is prevented from the damage of particles of impurity, the abrasion resistance of the seal ring is improved, and the serving life of the seal ring is prolonged.