Abstract: Reaction beam is the main reaction device in civil engineering test. When the stiffness ratio of the reaction beam to the specimen is small, the deformation of reaction beam cannot be neglected compared with that of the specimen. In this case, it has a great effect on the experimental results, especially on the pseudo-dynamic and hybrid testing method, which need to update the displacement online. However, there are few reports on the influence and compensation method of the deformation generated by the reaction beam. This paper discusses the influence of the reaction beam deformation on the tested seismic parameters through theoretical analysis. Furthermore, an online compensation method for the deformation of reaction beam during loading is established and verified by experiments.