The invention discloses an operating mechanism used in a Par4 parallel robot. The operating mechanism comprises a workbench big connecting arm, wherein a spherical hinge head is bonded on the workbench big connecting arm, a workbench small connecting arm is articulated with the workbench big connecting arm through a rotating shaft, a shaft sleeve and a bearing, a four-bar motion mechanism is formed by four articulated connecting arms, fan-shaped main gears are connected to the workbench small connecting arm by virtue of shafts, the fan-shaped main gears and the small connecting arm are positioned by virtue of pins, a working shaft positioning beam is articulated on the workbench small connecting arm by the bearing and a collar, a pinion shaft is articulated on the working shaft positioning beam and meshed with the two fan-shaped main gears which are fixed on the small connecting arm, and the degree of freedom of a motion platform is expanded to four degrees of freedom by adopting the movement characteristics of the four-bar mechanism. The mechanism has the advantages of simple structure, high precision, good stability, strong load bearing capability and the like.