A numerical method is used to analyze the nonlinear aerostatic stability for a 420m-main-span suspension pedestrian bridge. Based on aerostatic coefficients of the main girder measured by wind tunnel tests, nonlinear influences caused by structure and aerostatic load are included in this incremental double iteration method. Numerical results are presented as follows: (1) The structure deformation with wind speed show obvious nonlinearity, and the aerostatic instability form has the characteristic of space deformation of bending-twisting coupling significantly; (2) Comparing with the highway bridge, torsional stiffness from main cables accounts for greater proportion in long-span suspension pedestrian bridge. The critical wind velocity and the reason of aerostatic stability can be got by tracing the stress of the main cable; (3) Nonzero initial wind attack angle can reduce the critical wind speed;(4) Central buckles and wind cables can raise the critical wind speed. The aerostatic instability form applied the central buckles measures is bending-twisting coupling of main girder, and the aerostatic instability form applied the wind cables measures is local structural buckling due to the wind cables and hangers stress relaxation. Meaningful references are provided for the research of narrow bridge span aerostatic stability and enhance its aerostatic stability.