To reasonably characterize the stress state of corrugated steel web girders and determine the failure load, the stress state evolution pattern of corrugated steel web girders was investigated in this paper based on the structural stress state theory, relying on the model test of a continuous corrugated steel web composite box girder bridge with a span arrangement of (2.65 + 4.10 + 2.65) m. First, the generalized strain energy density was constructed as a characteristic parameter using the longitudinal strain of the test girder, and then the strain energy density?load curve was obtained. The Mann-Kendall criterion was applied to analyze the stress state curve and identify the characteristic points where abrupt changes occur in the stress state characteristics. The failure load of the corrugated steel web composite girder bridge was defined according to the law of quantitative to qualitative change. Furthermore, the working behavior of the structure was discussed by combining the strain-based stress state mode, the stress state sub-mode, and the energy ratio sub-mode. Finally, the application of calculating the characteristic load using this method in the design of corrugated steel web girder bridges was discussed. The results indicate that the Mann-Kendall criterion can identify three characteristic points of failure during the loading process of the corrugated steel web girder, and the entire stress process is divided into four stages: elastic, elastoplastic, failure, and continuous failure; the strain-based stress state mode, the stress state sub-mode, and the energy ratio sub-mode can all capture the stress characteristics of the corrugated steel web at different stress stages to varying degrees. For structural design, the failure load identified by the Mann-Kendall criterion can be adopted as the design load.