The utility model relates to a metallurgical technology field discloses a negative plate interval adjusting device's hydraulic system, include: hydraulic tank, constant voltage variable piston pump and the execution return circuit of establishing ties and setting up, carry out the return circuit including parallelly connected lift subloop and horizontal subloop, the lift subloop includes lift relief pressure valve, switching -over valve, one -way throttle valve and lift cylinder, horizontal return circuit includes horizontal relief pressure valve, proportional valve and horizontal oil cylinder, hydraulic tank is used for storing and provides the circulation required hydraulic oil, constant voltage variable piston pump is used for pump sending hydraulic oil. The utility model discloses a constant voltage variable piston pump, can adjust the hydraulic system flow in real time according to system's needs, and pressure stability, this hydraulic system's energy -conserving performance is good goes with control performance.