In order to reduce the self weight of railway line temporary beam buttresses and improve their construction convenience and reusability, this paper proposes the segmental-assembled steel temporary beam buttress. This kind of supporting pier is composed of three steel box segments, which are assembled through connecting plates and bolts. This article processed and produced a 1:4 scaled test specimen of this new-type buttress. The mechanical behavior and failure mode of the specimen was studied by applying a vertical static load equivalent to that transmitted by the temporary beam. According to the vertical load-displacement curve of the test, it can be seen that the specimen did not experience local buckling or overall instability during the loading process, and the overall structural performance is strong. The ultimate bearing capacity of the test component is 388kN. The assemble joints between adjacent steel box segments are the weak region of the structure. The construction difficulty for the 1/4 scale test specimen may lead to the asymmetric deflection. The main failure modes are the punching failure of the steel plate near the bolt hole and shearing failure of bolts caused by the continuous increase of the joint gap width. It is indicated that the bending resistance of the joint is relatively low and needs to be properly strengthened during the design process. This study provides important support for the design and application of new-type segmental-assembled temporary beam buttress.