The utility model provides an oil adding device under a pump. The device comprises an oil pipe short joint, a sacrificial anode fixed in the oil pipe short joint, a magnetized component fixed in the oil pipe short joint and a support component, wherein the support component comprises a support body and a first support used for supporting the support body, the first support abuts against the inner wall of the oil pipe short joint, the support body is columnar and arranged in the axis direction of the oil pipe short joint, the support body is sleeved with the sacrificial anode and the magnetized component in sequence in the axial direction of the support body, and liquid can flow into the oil pipe short joint through the inlet end of the oil pipe short joint and flow out through the outlet end of the oil pipe short joint. By arranging the sacrificial anode and the magnetized component, scale prevention treatment can be conducted to a fixed valve of an oil well pump through the sacrificial anode, wax prevention treatment and viscosity reduction treatment can be conducted to well liquid through the magnetized component, so that the pump detection period can be prolonged, and therefore the purpose of improving the well yield can be achieved.