The invention discloses an ultrafine grain rolling method of a large-size 45 steel bar, and relates to the field of machining, particularly to the ultrafine grain rolling method of the large-size 45 steel bar. The ultrafine grain rolling method comprises the following steps of design of rolling tools, specifically comprising design of rollers and design of guide plates: setting the rollers as double-curved surface circular table rollers, specifically, forming generatrices of the rollers through connecting two curves, and setting one surfaces of the guide plates as curved surfaces; constructionof a deformation zone: oppositely placing the curved surfaces of the two guide plates, placing the two rollers between the guide plates, and setting a region refined by the two guide plates and the two rollers as the deformation zone; and selection of rolling feeding manner: selecting reverse feeding rolling manner, and constructing an even-ovality deformation zone. According to the ultrafine grain rolling method of the large-size 45 steel bar provided by the invention, the double-curved surface circular table rollers and the curved surface guide plates are designed, the even-ovality deformation zone is constructed, and severe plastic deformation under the precondition of the Mannesmann effect of a central part can be remarkably inhibited.