Abstract: This paper presents an analysis of bearing capacity of FRP-concrete-steel tube composite square columns based on axial compression tests, focusing on the influences of hollow ratio, diameter-to-thickness ratio of inner steel tube, the number of FRP layers wrapped on the columns, etc. to promote their application in hydraulic structures. Based on our test data and previously reported data, we have derived calculation formulas and a corresponding model for simplified calculation of the bearing capacity varying with these three major factors. The results show that the bearing capacity of this composite structure is significantly improved due to its sandwich concrete that is highly confined by its inner steel tube and outer FRP tube. Under axial compression, the most significant factors are the confining degree of the FRP tube and the diameter-to-thickness ratio, and the bearing capacity increases with hollow ratio when the diameter-to-thickness ratio is kept nearly constant. The calculations of the formulas agree well with the results of axial compression tests.