An underground hydraulic automatic reversing rodless sand-sticking-prevention oil well pump is used for oil field production. The device makes a piston reciprocate through power provided by a ground hydraulic pump, and therefore produced liquid can be lifted. The pump comprises a power liquid pipeline, a power-lacking liquid pipeline, a reversing valve, the piston, a piston cylinder, a reversing needle and a check valve. The pump provides power through ground hydraulic pressure so that rodless production can be achieved; on one hand, power liquid transmits power, and meanwhile cooling lubrication is achieved; it is ensured that the pump efficiency is 85-90%. By means of the pump, the sand sticking phenomenon existing in ordinary hydraulic pumps is fundamentally changed, the pump inspection period is greatly prolonged, and the pump is suitable for various complex well situations such as later-period production of inclined shafts, deep wells and oil fields.