The invention discloses a double-tank control valve, which comprises a main control valve and an auxiliary valve, wherein the auxiliary valve is connected with the main control valve through a connecting joint; the main control valve comprises a water inlet, a water outlet, a salt sucking port and a drainage port; the main control valve further comprises an ejector and eight piston control valves; the eight piston control valves comprise a water inlet valve, a large bypass valve, a filtering back washing valve, a filtering bypass valve, a softening back washing valve, a softening positive washing valve, a water outlet valve and a salt valve; the eight piston control valves are driven by a piston to open or close water paths of a valve base upper port and a valve base lower port. A series double-tank system is controlled by adopting one control valve, so that the system can be controlled more flexibly, the cost is lowered, and the potential safety hazard of disorder caused by separate control on filtering and softening is avoided.