The invention discloses a rolling forming method for a 5CrNiMo steel thin-wall ring, which is used for rolling and forming a thin-wall ring with high dimensional precision and good texture and properties. In the technical scheme of the method, a 5CrNiMo steel pre-rolled compact heated to a deformation temperature and a preheated following-up die sleeve are firstly sleeved into a ring rolling mill, the following-up die sleeve is sleeved into the pre-rolled compact, and the two workpieces are positioned through a main roller, a core roller, a holding roller, an upper cone roller and a lower cone roller of the ring rolling mill; the ring rolling mill is started again, the core roller of the ring rolling mill does a feed motion toward the direction of the main roller along a radial direction, and the core roller and the main roller roll the pre-rolled compact with rolling force of 180KN-3800KN under the support of the following-up die sleeve together so that the pre-rolled compact is broadened along the radial direction at the speed of 6mm/s-12mm/s, and the wall thickness of the pre-rolled compact is gradually decreased; and the pre-rolled compact is rolled into a thin-wall ring after the deformation reaches 30%-50%. The minimum of the wall thickness of the 5CrNiMo steel thin-wall ring rolled and formed by adopting the method is 25mm, and the maximum of the depth to thickness ratio is 25mm. The ring is mainly used for rotor parts used in the field of special vehicles, and the like.