Abstract: A nonlinear instability mode of torsion for long-span cable-stayed bridges under aerostatic load is proposed, and nonlinear equations to solve equilibrium path are derived out. The theoretical analysis for entire process is made for stability of equilibrium path of aerostatic torsion. The effect of nonlinearity of pitch moment coefficient and module of rigidity of torsion on conditions of stability, instability points and instability mechanism is analyzed. Property of instability for aerostatic torsion is that the instability is extrema instability with jump. The condition of arising instability is related to shape of curves of pitch moment coefficient and module of rigidity of torsion. Only the curve of pitch moment coefficient has concave segments and the module of rigidity of torsion has bulgy segments. The instability of torsion can be perhaps arisen. If there are no concave segments in the curve of pitch moment coefficient and no bulgy segments on the curve of module of rigidity of torsion, the equilibrium path is stabile and the instability of aerostatic torsion can not be arisen. The module of rigidity weaken of torsion is one of causes of instability of aerostatic torsion. The equivalent module of rigidity weaken of torsion brought by the nonlinearity of pitch moment coefficient is also one of causes of instability of aerostatic torsion. While an initial attack angle is biggish the effect of equivalent module of rigidity weaken ought to be especially noticed. A new formula of critical wind velocity for instability is presented. The formula is uniform because it contains the result of nonlinear instability and linear theory. The formula is compact because it is formally same as the formula of linear theory. For linear module of rigidity or pitch moment coefficient physical significance of a critical point is unambiguous which is expediently obtained by illustrating. The new formula of critical wind velocity reflects the effect brought by the initial attack angle. While the initial attack angle augmentes the critical wind velocity minishes.