The invention provides an aperture expanding device for a paper can in a paper can forming machine, belonging to the technical field of paper can forming equipment. The 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; when the connecting rod rotates, the expanding-converging mechanism can move in a face-to-face or back-to-back direction by using the arc tableting piece I and the arc tableting piece II as connecting rods; a forming cavity matched with the paper can is formed between the outer sides of the arc tableting piece I and the arc tableting piece II and the inner side of the cylinder body when the connecting rod rotates; therefore, the apertures of the paper can are unified, and a function of automatically expanding apertures is achieved.