The invention relates to a constant pressure variable pump system and an energy-saving method. It is composed of several pump unit units, each of which is connected to an oil supply system. The pump unit consists of a constant pressure variable pump, a control slide valve, Relief valve, electromagnetic reversing valve, check valve, ball valve 1, ball valve 2, and ball valve 3 are composed of constant pressure variable pump, overflow valve, check valve, ball valve 1, and the oil supply system are connected in sequence, and the constant pressure variable pump is connected in sequence. A control spool valve is connected, the overflow valve is controlled on and off by an electromagnetic directional valve, the control spool valve is connected to the oil supply system through ball valve 2, and the electromagnetic directional valve is connected to the oil supply system through ball valve 3 and ball valve 2 in turn. The advantages of the present invention are: through the improvement of the pump unit and the addition of ball valves, the single control of the pump unit is realized; through the improvement of the control process, the energy waste of the constant pressure variable pump is reduced when the pump is in standby, and the oil flow rate is extended. of life.