The invention provides an improved aperture expanding device of a paper can used in a paper can forming machine, belonging to the technical field of paper can forming equipment. The aperture expanding device comprises a cylinder body, a connecting rod, an arc tableting piece I, an arc tableting piece II and an expanding-converging mechanism, wherein the cylinder body is cylindrical and the lower end of the cylinder body is opened; the lower end of the connecting rod is positioned in the cylinder body; the upper end of the connecting rod extends out of the upper end of the cylinder body and is connected with a driving piece which can drive the connecting rod to rotate; the expanding-converging mechanism is positioned between one side of the connecting rod and the arc tableting piece I and between the other side of the connecting rod and the arc tableting piece II; a strip-shaped material returning hole which penetrates along an axial direction is formed in the side part of the cylinder body; a material returning ring matched with a forming cavity is arranged on the top of the forming cavity; the material returning ring is provided with a protruded handle; the handle penetrates through the material returning hole and extends out of the cylinder body; therefore, the apertures of the paper can are unified, and functions of automatically expanding apertures and removing a paper tube are achieved.