The invention provides an automatic plastic bag cutting device. The automatic plastic bag cutting device comprises a conveying device and a cutting device, wherein the cutting device is arranged at the right side of the conveying device; the conveying device comprises a conveying roller, a tension force regulating device and a pressure regulating device; the conveying roller and the pressure regulating device are sequentially arranged at the left and right sides of the tension force regulating device; the tension force regulating device comprises a slider, a guide rail and two tension rollers;the two tension rollers are arranged in an up-down manner and are fixed to the slider which is positioned below the tension rollers and matched with the guide rail by virtue of a support; the slideris provided with a locking device; the locking device realizes locking by virtue of a friction force; the cutting device comprises a feeding roller, an incomplete gear, a thermal cutting knife and a cutting groove; one end of the feeding roller is fixed to a platform by a support; the other end of the feeding roller is provided with an inner meshed gear; and the thermal cutting knife is positionedat the right side of the feeding roller. The automatic plastic bag cutting device provided by the invention can guarantee accurate cutting and smooth surface of a plastic bag in a machining process,and prevents occurrence of defective products.