The invention discloses a hydraulic mud pump which is provided with an oil inlet P, an oil return opening T, a pulp inlet V and a high-pressure pulp outlet H, and comprises a pressure cylinder and a hydraulic operated directional valve assembly. The pressure cylinder comprises a low pressure cylinder, a low pressure piston, a left high pressure plunger, a right high pressure plunger, a left high pressure cylinder and a right high pressure cylinder; a first oil inlet passageway, a second oil inlet passageway, a first control passageway and a second control passageway are arranged on the low pressure cylinder; an annular communicating groove is formed in the low pressure piston; when the low pressure piston leftwards moves to a position completely covering a first through-flow groove, the first oil inlet passageway and the first control passageway communicate through the communicating groove, the hydraulic operated directional valve assembly is controlled to reverse, and the low-pressurepiston is braked through a sealed left piston cavity; and when the low pressure piston rightwards moves to a position completely covering a second through-flow groove, the second oil inlet passagewayand the second control passageway communicate through the communicating groove, the hydraulic operated directional valve assembly is controlled to reverse, and the low-pressure piston is braked through a sealed right piston cavity. The hydraulic mud pump not only is simple in structure, but also has no need to be electrically controlled, and has low noise.