The invention discloses a belt detachment tool for a pumping unit in an oil field, and belongs to the field of petroleum machinery. The belt detachment tool comprises a driving device, a fixing device, a belt carrying device and an adjustment device. The driving device comprises a rotary hydraulic cylinder, an outer spline shaft and an inner spline shaft. The fixing device comprises an inner movable sheet bar and a clamping ring. The belt carrying device comprises an outer movable sheet bar and a static sheet bar. The adjustment device comprises a bearing platform, a thrust bearing, a screw nut, a lead screw, a bearing seat, a second bearing and a bearing cover. The movable sheet bars and the static sheet bar can break away and get close to realize the installation and detachment of a belt. The belt detachment tool can be installed on a belt wheel only through one bolt, so that operation is simple. A motor does not need to move in the detachment and installation process of the belt, it is guaranteed that the pre-tightening force of the belt is basically constant, the motor works stably, an operator only needs to drive a manual hydraulic pump and does not need to take part in other processes, safety risks are reduced, and operation efficiency is improved.