The invention discloses a hybrid power system for recovering the potential energy of a movable arm of an excavating machine. A check valve I, an auxiliary box, a motor, an electricity storing unit, a motor controller, a pressure sensor I, a pressure sensor II, a hydraulic motor, a clutch, a main controller and a check valve II are added on the basis of the conventional excavating machine; and an engine, the motor, the hydraulic motor and a master variable pump are combined into a power system through the auxiliary box. When the movable arm is lowered, oil flows back in two channels, a first oil backflow channel is connected into the hydraulic motor, a second oil backflow channel is connected into the master variable pump, and the hydraulic motor and the master variable pump are used for transmitting power to the power system respectively. The motor has an electricity generating function; by controlling the motor, the function of generating electricity through absorbed energy or providing driving energy through output energy can be realized, and the aim of energy conservation can be fulfilled; and the hybrid power system has the advantages of a small number of used elements, small element specification and low cost.