Steel structure reaction frame is an important test instrument to carry out static and quasi-static tests of civil engineering structures. In the test process, the mechanical properties of steel structure reaction frame will directly affect the reliability of the test results. Therefore, in this paper, the steel structure reaction frame involved in the whole process of pseudo-static test loading is taken as the research object, the key stress position of the steel structure reaction frame is arranged by measuring points, Midas is used to carry out finite element analysis of its stiffness changes, and verify the correctness of the finite element model. The results show that the maximum difference of stress at the same position is 28.08% before and after horizontal loading, and the change of stress at each measuring point of steel structure reaction frame is small. The maximum stress difference calculated by Midas between steel column and the corresponding test point 21.64%, and the maximum stress difference between steel beam and the corresponding test point 23.97%. Under vertical load, the error between the Midas displacement calculation results and the reference manual's calculation results of beam end consolidation is 18.92%. Through theoretical and finite element error analysis, it is found that the Midas calculation model is more consistent with the actual boundary conditions, and the displacement calculation results are more accurate. Select the typical design specification to check the stiffness of steel beam, it is found that steel beam stiffness conditions are far less than the limit of the specification, steel structure reaction frame meets the design and large test loading accuracy requirements. The results of this paper can provide reference for the design and optimization of steel structure reaction frame system in engineering structure laboratory.