Tower structures are important device in the chemical equipment. They were lifted up by pulled down shearlegs. Tower structures' load is complex and changeful within lifting up. It is multifarious to analyze and to compute. At present, because people always adopts experiential or static analyzing and computing, one neglects changed circumstance and can adopt irrationally kinds of measures. It leads to break down tower structures and be lost in lifting up. This paper deduces computing formulae of danger case and provides basis for design for technics of lifting up according to analyze movement.