The invention discloses an oil supplement pump assembly and a closed variable-displacement piston pump. The oil supplement pump assembly comprises an oil supplement pump shell and a cycloid inner-outer rotor pair which is mounted in the oil supplement pump shell. The cycloid inner-outer rotor pair comprises an outer rotor and an inner rotor, wherein the outer rotor is mounted in the oil supplement pump shell, and the inner rotor is arranged in a middle cavity of the outer rotor and engaged with the outer rotor. A shaft hole is formed in the middle of the inner rotor. At least a left oil cavity and a right oil cavity are formed between the outer rotor and the inner rotor, and the volumes of the left oil cavity and the right oil cavity can change along with rotation of the inner rotor and the outer rotor. The oil supplement pump shell is provided with an oil hole and an oil suction port, and the oil hole and the oil suction port communicate with the two oil cavities. The closed variable-displacement piston pump comprises a main pump body and the oil supplement pump assembly, wherein a kidney-shaped groove is formed in the rear end face of the main pump body and located besides a spindle, the kidney-shaped groove right faces the two oil cavities of the oil supplement pump assembly, and an oil supplement hole communicating with a high-low pressure oil cavity of the main pump body is formed in the bottom of the kidney-shaped groove. The oil supplement pump assembly and the closed variable-displacement piston pump are more reasonable in structure and easy to operate, the requirement for the skills of operators is low, and resistance loss generated by working oil of a main pump system in the working process is smaller.