Based on the unified strength theory, the axial ultimate bearing capacities of composite L-shaped steel tube short columns filled with steel-reinforced concrete (CLSC-SRC) under three-dimensional stress were analyzed in this paper. According to characteristic of section shape, the column was firstly divided into one rectangle and one square steel tubular columns filled with steel-reinforced concrete. Then, considering the influence of width to thickness ratio on steel tube and a concrete strength reduction factor was introducect with scale effect. Because nonuniform constraint of the long and short edge of the steel tube was equivalent to the uniform constraint of the circumferential direction, a formula for calculation of axial compressive bearing capacity of CLSC-SRC can be developed. Finally, the formula for calculating the compression bearing capacity of the steel bone L-shaped concrete-filled steel tubular long columns was proposed referring to the code for design of steel structure. The proposed fomula was validated by comparing the calculation results with the corresponding test data. Results show that the proposed foumulations are correct and reasonabce. The effects of the parameters and Pull pressure ratio of Material and Osseous rate were obtained in the theoretical analysis. More importantly, the proposed formula has good applicability and extensive applications, offering a useful supplement to existing formulas obtained from experiments.