Metal support is an important support material for underground space surrounding rock control engineering. It has the defects of low construction efficiency, high support cost, passive support and other defects. The mathematical expression of the theory of the pressure-filled elbow expansion (PFEE) is deduced. It is pointed out that the PFEE force is proportional to the curvature of the elbow, the effective perfusion pressure and the diameter of the elbow. Again, combined with the on-site pouring process and the extension mechanical characteristics of the flexible hose, the extended composite flexible support (ECFS) is proposed and defined, and its construction process is designed. It has the advantages of high construction efficiency, high support strength, low support cost, active support of surrounding rock and etc. Finally, the similarity simulation test bench on ECFS is designed and processed to test the minimum perfusion pressure and the value of the active support vertical component of the ECFS, and the mathematical expressions are derived. The test values are basically consistent with the calculated results, which verifies the PFEE theory and the feasibility of engineering application of the ECFS.