The utility model relates to a variable-diameter hydraulic casing centralizer which comprises an upper connector, a central pipe and a lower connector. The central pipe is in threaded connection with the upper connector and the lower connector, the central pipe is sleeved by a fixed piston in a sealing mode, the fixed piston is in threaded connection with the upper connector, the portion, under the fixed piston, of the central pipe is sleeved by a hydraulic cylinder in a sealing mode, an annular hydraulic cavity is formed among the inner wall of the hydraulic cylinder, the lower end of the fixed piston and the outer wall of the central pipe, liquid inlets are formed in the circumferential lateral wall of the central pipe, and the central pipe is sequentially sleeved by a thrust connector, a conical body and a sectioned centralizing body. A withdraw preventing ring is arranged between the conical body and the central pipe, a first conical inclined face is arranged on the outer wall of the lower end of the conical body, a second conical inclined face matched with the first conical inclined face is arranged on the inner wall of the upper end of the sectioned centralizing body, and the lower end of the sectioned centralizing body is in threaded connection with the lower connector. The downward resistance of the centralizer is reduced, a lateral drilled horizontal well casing is kept in the middle, and the cement loss is avoided.