The invention provides a packing box folding device which is used for folding paper. The packing box folding device comprises a machine frame, a shifting pressing device and a folding device. The shifting pressing device is installed on the machine frame and comprises a driving device and a pressing plate, and the pressing plate comprises a folded edge, is in transmission connection with the driving device and performs linear reciprocating motion under the driving of the driving device. The folding device is fixedly connected with the machine frame and comprises a locating block, the locating block comprises a folding plane, the folded edge corresponds to the folding plane, the pressing plate pushes the paper to move towards the folding device under the driving of the driving device, and the paper is folded to form an angle of 90 degrees through the matching of the folded edge and the folding plane. The packing box folding device has the advantages of being reasonable in design, simple in structure, not only high in automation degree, but also capable of meeting the large-scale production requirements and guaranteeing product consistency and improving product quality.