An automatic locking device for assembling a part to a workpiece is provided. The device includes a feeding mechanism, a fixture mechanism, a loading and unloading mechanism, a lifting mechanism, a rotating mechanism, and a locking mechanism. The feeding mechanism feeds the part to the fixture mechanism. The loading and unloading mechanism places the workpiece on the part. The lifting mechanism lifts up the fixture mechanism together with the part and the workpiece. The rotating mechanism rotates the fixture mechanism. The locking mechanism locks the part to the workpiece after the part and the workpiece are rotated by the rotating mechanism. The automatic locking device integrates multiple functions in a single station, reduces the number of the work stations and operatives, lowers the cost, and improves accuracy of the assembly. An assembling apparatus including the automatic locking device is also provided.