The embodiment of the invention provides a method and a device for determining the convexity of a loaded roller gap. The method comprises the steps of obtaining the type and specification of steel; determining the convexity correction factor of each finish rolling rack according to the type and specification of steel; obtaining the convexity of the loaded roller gap, and applying the total rollerbending force and the special rolling force to each roller, the convexity of a no-load roller system of a rolled piece and the interface of a working roller and the convexity of the no-load roller system of the working roller and the interface of a support roller; and determining the convexity of the loaded roller gap according to the convexity correction factor of a roller gap, the convexity of ano-load roller gap, the total roller bending force and the special rolling force which are applied to each roller, the convexity of the no-load roller system of the rolled piece and the interface ofthe working roller and the convexity of the no-load roller system of the working roller and the interface of the support roller. Thus, the convexity of the loaded roller gap of each finish rolling rack is corrected in real time according to the convexity correction factor of the roller gap of each finish rolling rack, so that the accuracy of the convexity of the loaded roller gap is increased, stable rolling can be guaranteed, and the quality of strip steel products is guaranteed.