The research has demonstrated that the stress and strain distribution on the surface of the ST12welded I-section steel bracing members gained by numerical analysis is in accordance with the true distribution. In this paper, three critical plane damage models, established based on the relationship between the strain and the low cycle fatigue life, are used to simulate the damage and predict the low cycle fatigue lives of the ST12welded I-section bracings. The results show that, the location of the crack initiation can be predicted accurately, and the predicted lives are all less than the test ones. The empirical formulas are regressed and compared with the empirical formula obtained by the test results. It is verified that the main factors affecting the low cycle fatigue life are the flange width-thickness ratio b/t and the slenderness λ. Under the premise of meeting the design requirement of "no buckling under frequent earthquake" and the allowable range of standards, it is suggested that small b/t and large λ should be adopted in the brace design.