The invention provides a method for designing a conjugated blade surface chopping shear. The method comprises the first step of establishing a fixed coordinate system and two movable coordinate systems fixedly connected with an upper roller and a lower roller, the second step of constructing a shear blade pair conjugated blade shape of a shearing tool inlaid in the upper roller and a shearing tool inlaid in the lower roller, the third step of expressing the rake faces of the shearing tools with a cylindrical surface sectional helical line and end face sectional curves, and establishing a conjugated model of the rake face, conjugated with the cylindrical surface sectional helical line and the end face sectional curve, of the tool of the upper roller, the fourth step of solving an end face sectional conjugated curve of the rake face of the tool on the upper roller with a shearing overlapping region as a conjugated region, and the fifth step of establishing a tool coordinate system and establishing an end face conjugated sectional optimization model of the rake faces of the tools with the maximum normal error of the end face sectional curve of a conjugated pair to be minimized as the target. By the adoption of the shearing tool designed through the method, the end sectional curve of the rake face of the tool on the upper roller and the end sectional curve of the rake face of the tool on the lower roller are identical; it is guaranteed that cutting gaps are consistent by adjusting the starting phase positions of the tools of the upper roller and the lower roller according to the thickness of a sheared steel plate and the requirements for technological backlashes, interference between blades is avoided, and the tools are good in durability.