The invention discloses a method and a device for controlling a rolling mill to roll a finished steel material. The method comprises the following steps: acquiring information of a virtual welding line, wherein the information of the virtual welding line includes the thickness of each roll of to-be-rolled steel material, the thickness of the finished steel material and/or the width of each roll to-be-rolled steel material; determining the weight of the finished steel material according to the information of the virtual welding line; determining the virtual welding line of the finished steel material according to the weight of the finished steel material, wherein the virtual welding line is a cutting position of the finished steel material on each roll of to-be-rolled steel material; determining control parameters according to the distance from the virtual welding line and a joint point, wherein the joint point is a joint position of the inlet region of the rolling mill and a rolling mill region; and changing the specifications of each roll of to-be-rolled steel material according to the control parameters to obtain the finished steel material, wherein the finished steel material isa steel material with a preset specification and a preset weight. The method disclosed by the invention can solve the problem that the finished steel material cannot be rolled directly according to weight requirements, and can be used for obtaining the finished steel material with the preset specification by direct rolling according to the preset weight without subsequent slitting treatment, so steel materials and time are saved.