To improve the yield rate of steel plates, a method of slab size design is proposed based on maximum yield rate of plates. It includes material transformations algorithm and optimization of slab selection. Many factors are considered in the model covering the equipment capacity, spread rate, compression rate, burning loss and cutting loss. Initially, all possible types of slabs are obtained through material transformations algorithm after the order input. This allows the eventual determination of the slabs of the largest yield rate through the optimization of slab selection. The experiments prove that the model can reduce a number of materials and lessen the pressure of the inventory. An effective increase of plate yield can be expected to improve the economic efficiency of enterprise.