The tube conical nosing process can be resolved into three deformation instantaneous states, namely, free bending - inflection straight - forced conical nosing. The right transformation has important effects on forming quality of nosing parts. The conversion conditions from free bending to inflection straight and nosing failure causes were derived based on the energy method. The calculating formulate for critical semi-cone angle was proposed and verified by finite element simulation. It was found: the bigger the hardening exponent, the larger the value of critical semi-cone angle; the geometric parameters have litter effect on the value of critical semi-cone angle while the materials are same. The thin-walled nosing parts with large semi-cone angle would wrinkle and the multi-pass forming process should be adopted. When the semi-cone angle of part is greater than the critical angle, it leads to curl.