With the application of suspension bridges with double main spans, the aerostatic stability should be studied in order to maintain the safe operation. A procedure based on aerostatic load increment and double iterations was programmed and applied to analyze the aerostatic stability of three suspension bridges with double main spans. The characters of aerostatic instability for suspension bridge with double main spans were gained by comparing the instability modes between the suspension bridges with single and double main spans. There are two aerostatic instability modes for suspension bridge with double main spans, which include bilateral anti-symmetric instability mode and unilateral non-symmetric instability mode. Meanwhile, the mechanism of the two modes was discussed in detail.