For the fracture defects at the bottom of punch in the onestep forming of the conical parts, the forming characteristics and forming mechanism were analyzed. Then, the maximum explorator displacement location of mass poiut on the cone wall was defined as the key point, and the calculated conditions of conical parts without fracture at the bottom of punch during the onestep forming  to limit condition were obtained. Moreover, the onestep forming process of conical parts was simulated by the Dynaform software. Through comparing the calculated values with simulated values, the relative errors were obtained, and the applicability of computing conditions was analyzed. The experiments were conducted to verify the applicability of calculated conditions, through comparing the calculated values with experimental values, the correction factors of calculated conditions were obtained, and the relative errors were all less than 22% after correction. Subsequently the calculated conditions after correction are more applicative for judging no fracture at the bottom of punch.