A positioning device includes a base, a supporting member, a fixing member, a first electromagnetic clutch, an engaging member, and at least one guide member. The base defines at least one hole. The supporting member is configured to support a workpiece. The supporting member and the fixing member are at the opposite sides of the base. The first electromagnetic clutch is disposed on a side of the base adjacent to the fixing member. The engaging member configured to be attracted by the first electromagnetic clutch is disposed on a side of the fixing member adjacent to the base and corresponds to the first electromagnetic clutch. An end of the at least one guide member is fixed to the fixing member, and an opposite end of the fixing member passes through the at least one hole and is fixed to the supporting member.