The invention discloses an automatic tensioning device of a stocker fork chain. The automatic tensioning device of the stocker fork chain comprises a chain fixing base, a first toothless chain wheel, a second toothless chain wheel and a tensioning base, wherein the chain fixing base is fixed to an inner fork board; the first toothless chain wheel is fixed to a middle fork board; the second toothless chain wheel is fixed to an outer fork board; the tensioning base is fixed to an installation board and provided with a tensioning shaft; one end of the tensioning shaft is provided with a tensioning spring and a locking nut; the other end of the tensioning shaft is connected with the fork chain; the other end of the fork chain sequentially penetrates through the second toothless chain wheel and the first toothless chain wheel and is connected to the chain fixing base. The automatic tensioning device of the stocker fork chain not only is small in occupied space, but also almost does not need manual tensioning.