A positioning device includes a base and two positioning mechanisms oppositely positioned on the base to position a workpiece. Each positioning mechanism includes a driver positioned on the base, a mounting member mounted on the base, a gear rack, a gear, and a positioning member. The mounting member defines a sliding groove. The gear rack slidably is received in the sliding groove and connected with the driver. The gear is rotatably connected with the mounting member and is meshed with the gear rack. The positioning member is fixedly-connected with the gear. The driver drives the gear rack to slide along the sliding groove to make the positioning member rotate and outstretch into an interior of the workpiece. The drivers drive two positioning members apply two opposite forces on the workpiece to positioning the workpiece.