Case-study is carried out on a large-scale thermal power plant structure with and without energy dissipating bracing system. The plant is designed for construction in a region of high seismicity in China. The project is 8-story 40.77 meters high, 6-bay RC frame in the longitudinal direction. The seismic design proposed to add X-type steel bracing in the longitudinal frame and to adopt a new type of lead-alloy damper of energy-dissipation bracing system. The study is to carry out elastic and nonlinear time history analysis to variable levels of earthquake input so to provide the design with the information of the effectiveness of the damper installation. The paper presents the results of analytical evaluation on the longitudinal frame's seismic responses. In order to compare the damping effects of different installations, four structural models with and without bracing and dampers are used to conduct nonlinear dynamic response analysis subjected to three different levels of ground motion input, respectively. The peak acceleration levels correspond to the frequently occurred earthquake, moderate earthquake in seismic fortification intensity (in 8-degree area), and expected once in the lifespan the rare-intensive earthquake according to Chinese Code for seismic design of buildings [1]. The seismic performance of the four structural models is evaluated based on the analysis results (such as the vibration period, earthquake force, story shear force, floor displacements, dynamic amplification factors, etc). It is found that the installation of the lead-alloy dampers are effective to the steel bracing frame structure subjected to the rare-intensive earthquake and it reduces significantly the stress in the frame structure. Setting up the dual seismic resistant system, it can be effectively controlled the deformation of the frame structure and the damage of main structural elements. Based on the investigation, some suggestions are given in this paper towards the optimal structural design.