The invention relates to an assembly process for eccentric H-shape steel. The process is realized in a way that: widening the side with a smaller flange plate width by using a clamping fixture so that the width is the same as that on the other side, positioning the web plate in the center of the assembling machine, and converting the eccentric H-shape steel into non-eccentric H-shape steel, thereby realizing the mechanized production on the H-shape steel assembling machine. The clamping fixture comprises a backing plate and a check block, wherein the check block is welded on the upper part of the backing plate. The process can greatly enhance the efficiency, save the labor force and ensure the quality of workpieces. By using the clamping fixture, the eccentric H-shape steel is converted into non-eccentric H-shape steel, thereby achieving the goal of mechanized production on the H-shape steel assembling machine. The clamping fixture has the advantage of simple structure, can adapt to H-shape steel with different specifications through adjustment, and can not produce material waste.