The invention relates to the structure test field and especially relates to an assembling-type double-knife-hinge supporting seat system suitable for the pressure tests of rectangular steel tubes withdifferent sizes. The system comprises four-sided inverted-L-shaped clamps for fixing a component, a connecting bottom plate for fixing the component, an intermediate plate provided with an orthogonalgroove, and a lower portion connecting plate having a knife hinge on a surface. The two sides of the four-sided inverted-L-shaped clamp are connected to long bolt rods, a nut connected to a bolt canbe adjusted, the rectangular steel tubes with the different sizes are fixed and the relative position of the rectangular steel pipes and the connecting bottom plate is changed, and then, the purposesof axial compression and quantitative eccentric compression are achieved. The fixed component connecting bottom plate and the lower portion connecting plate which are provided with the knife hinges are cooperated with the intermediate plate provided with the orthogonal groove so as to realize bidirectional hinge supporting seat. A whole supporting seat device belongs to pure assembling connectionand has the series of the advantages of a wide suitable range, easy dismounting, high adjustable performance and the like.