The invention discloses a fast assembling method for a supporting roll of a rolling mill and belongs to the technical field of rolling mill equipment maintenance. A radial positioning principle is adopted, any end point of the roll diameter part of the supporting roll of the rolling mill serves as a positioning point, the positioning point is positioned through radial projection positioning, and the positioning point is located on a radial; and a same-side end point of the outer hole diameter part, parallel to the roll diameter part defining the positioning point of the supporting roll of the rolling mill, of a bearing hole of a bearing base serves as a tangent line point, the tangent line point is tangent to the radial transmitted by a radial transmitting device all the time, it is ensured that the central axis of the supporting roll of the rolling mill coincides with the central axis of the bearing hole of the bearing base all the time, and therefore the position relation between a roll diameter inner sleeve of the supporting roll of the rolling mill and a bearing in the bearing base is accurately determined, and assembling is completed rapidly and accurately. According to the fast assembling method for the supporting roll of the rolling mill, fast assembling of the supporting roll of the rolling mill and the bearing base is achieved, the phenomena that the bearing is knocked to be damaged and the roll diameter inner sleeve is scratched in the assembling process of the supporting roll of the rolling mill are effectively avoided, the working efficiency is improved, the service life of the roll diameter inner sleeve of the supporting roll of the rolling mill and the bearing is prolonged, and cost is reduced.