The conventional method of bending the large crankthrow was investigated by computer simulation combined with manufacturing trial, and the typical forging defects, such as constricted waist, folded cracks and horn mouth on forged blank were analyzed. On the basis of these results, a novel forging shape of preformed blank was proposed using anti-transformation method by computer simulation. The FEM simulated results show that all the above defects can be avoided by carrying out the novel method, furthermore, the maximum resistance of the novel bending deformation was reduced to 72% of conventional process, and the weight of forging blank can be decreased by 15%. Finally, the optimum forging shape was applied to the actual process, the simulation results were confirmed by manufacturing trial, and qualified forged piece was gained. The FEM model established can be used for further optimization of other types of crankthrow.