The invention relates to a displacement variable oil-well pump. A pump cylinder, a connecting
pipe, an oil
inlet valve and a small pump are sequentially connected; a large
plunger, an oil outlet valve and an uncoupling device are sequentially connected, an oil inlet is arranged between the oil outlet valve and an upper body of the uncoupling device, the
diameter of the upper body of the uncoupling device is less than the internal
diameter of the large pump cylinder, the inner
diameter of the connecting
pipe is more than that of the large pump cylinder to ensure that the joint of the lower part of the large pump cylinder and the upper part of the connecting
pipe generates a releasing ledge, and the upper body of the uncoupling device and the
lower body of the uncoupling device can be separated at the releasing ledge. Due to the combination of the large cylinder and the connecting pipe, a releasing ledge of the uncoupling device is formed, and the displacement of the oil-well pump can be changed by adopting the common uncoupling device; because the outer diameter of the
lower body of the common uncoupling device is more than the internal diameter of the releasing ledge, only the uncoupling device is released after lifted to the releasing ledge, and the uncoupling device can be released as long as being lifted to the releasing ledge. The invention can be used for reliably changing the displacement and prolonging the
oil well production period, and has remarkable economic benefit.