A volume-varied balancer for purifying blood is provided, wherein two membranes (3, 4) are set in the intracavity of the cavity (2) with interval. The intracavity of the cavity (2) is divided into a first cavity (5) and a second cavity (6) with the first membrane (3) and the second membrane (4), and a silicon oil cavity (7) is formed between the two membranes (3, 4). The silicon oil cavity (7) communicates with the intracavity of a piston pump (9) by a silicon oil conduit (8). The piston pump (9) is fixed on a bracket (1). A conduction through-hole (2c) is provided on the cavity wall of the first cavity (5) and connected with the inlet joint of a first three-way solenoid valve (10) via a pipeline (16). A conduction through-hole (2d) is also provided on the cavity wall of the second cavity (6) and connected with the inlet joint of a second three-way solenoid valve (11) by the pipeline (16). Ref. Fig. 4
