The concept of depth limit for the method applied to sinking of shaft lining by means of floatation in shaft boring that is widely utilized in mine construction in China was put forward. The mechanical state of shaft lining filled part of water when its pan touching the base rock of shaft bottom was analyzed. Contacting with the technical characteristic of shaft lining sinking by means of floatation

the calculation model of shaft lining filled part of water in mud before fixed

as a self-gravitating bar in liquid

was set-up. Based on energy methods as well as the principle of structural stability theory and hydromechanics theory

the analytical equation for vertical structure stability critical height of shaft cylinder filled part of water was calculated. A typical example was persuasively analyzed. The attention to use the equation was emphasized in shaft design and its construction. Finally

its further research contents and direction are pointed out.