A calibrator for oil pumping unit belt mounting comprises a nut, a large pressing cap, a pressing plate, a laser generator, a linked switch, a body, a compensating spring, a return spring, spring clamping pieces, an intermediate sleeve, sunk screws and rubber gauge plates. The upper portion of the body is cylindrical, a large cylindrical cavity and a small cylindrical cavity which are mutually communicated are arranged in the body, an inverted 'U'-shaped support is arranged on the lower portion of the body, the compensating spring and the laser generator are mounted in the small cavity, the intermediate sleeve is in threaded connection with the bottom of the large cavity, the return spring is mounted on the upper portion of the large cavity, the linked switch with a slope and with a 'V'-shaped groove arranged at the lower end is arranged at the center of the large cavity, the pressing plate sleeves the linked switch and presses the laser generator, the large pressing cap provided with a center hole is in threaded connection with the body, and the nut outside the large pressing plate locks the linked switch. Besides, the rubber gauge plates are fixed on the symmetrical spring clamping pieces on two sides of the support on the lower portion of the body respectively, and the spring clamping pieces are fixed at the bottom of the support by the sunk screws. The calibrator for oil pumping unit belt mounting is simple in structure, convenient to operate and cost-reducing and benefit-increasing, the service life of a belt is prolonged, and labor intensity of workers is relieved.