To study the static stability of suspension bridges under different simplified models and comprehensively analyze the entire instability process and failure modes, based on the mechanical characteristic that the instability of main tower results in the instability of the entire bridge, a full-bridge multiscale model and a single-tower solid model were established by ABAQUS, taking Nanjing Xianxin Road Yangtze River Bridge under construction as the project background. Linear stability coefficients, double nonlinear load coefficients, linear buckling modes, and failure modes were analyzed and compared. Results show that the double nonlinear stability safety coefficients were greatly reduced compared with the linear stability coefficients, and nonlinear stability calculation should be necessary for long-span earth-anchored suspension bridges. The linear stability coefficients of the full-bridge multiscale model were slightly larger than those of the single-tower solid model, while it was opposite for the nonlinear load coefficients. The stability results of the simplified single-tower model could not fully represent the real situation. The concrete principal compressive stress and reinforcement stress near the bottom of the tower both reached standard strength values when the main tower experienced nonlinear instability, indicating that the material yield led to the structural instability. The tower leg on the leeward side failed in the typical compression-flexural failure mode, while that on the windward side failed in the compressive-flexural-torsional failure mode. With the development of the thin-walled structure, more attention should be paid to stability when the strength requirements are met. The research can provide reference for the design and model simplification of long-span earth-anchored suspension bridges.