Variable displacement piston pump for loading machine

The invention discloses a variable displacement piston pump for a loading machine. The variable displacement piston pump for the loading machine comprises a variable pump 7, a flow piston 6 and an external control valve component 2, wherein the external control valve component 2 enables a servo valve component 3 to work on the right under action of spring force and a support point, and then a K port is communicated with an L port, pressure oil at an outlet of the variable pump 7 acts on a small cavity in the flow piston 6, and simultaneously the pressure oil at the outlet of the variable pump 7 sequentially passes through the K port, the L port, a C port, a B port, an F port and an E port, and then enters a large end of the flow piston 6. The variable displacement piston pump for the loading machine is designed for achieving the purpose of enabling an existing loading machine open type hydraulic system to achieve energy conservation during each work process, and has following several control functions of achieving positive flow, being sensitive to loads, keeping power constant and cutting off pressure.