The design of underground structures requires planning and an adequate understanding of the geotechnical conditions of the area that is being excavated. The selection of the engineering form and size of underground excavations, stability analysis and design of the support system depend on in situ stresses and geomechanical parameters of the rock mass. In recent years, the back analysis method has been widely applied in geotechnical engineering especially in the underground works. The method is based on the required input physical and mechanical informations, and can be divided into displacement back analysis method, stress back analysis method and coupled back analysis method. The back analysis applied in this research based on the displacement back analysis method. In this research work the mechanical properties of the rock mass and the initial stresses were obtained from back analysis of the displacement data. Determination of the rock mass strength parameters is very difficult especially in weak rocks. Design of structures on or in the rock will encounter fewer problems if the rock mass properties are available. In situ tests and back analysis based on the acquired data from instrumentation can be used to determine the strength properties of rock mass. Back analysis can be very important in this regard and is less expensive than in situ tests. The Isfahan-Shiraz railway tunnel No.1 is located in a high stress zone of Sanandaj-Sirjan. Before excavation the tunnel, the geotechnical and geophysical studies has not performed to identify the type of rocks and water condition of the rock mass. The tunnel is in the Shemshak formation belonging to the Jurassic period and the geologic formations are often contain coal shale and sandstone lenses. Due to the large deformation, the convergence tapes have been installed during and after the excavation. In this research, for the first time, optimized direct back analysis using genetic algorithm was performed for Isfahan-Shiraz railway tunnel No. 1 by three-dimensional finite difference software (FLAC 3D ). The elastic modulus, stress ratio, cohesion and internal friction angle have been determined by applying a new method in back analysis. In this method, displacement was assumed as a function of the unknown parameters. The differential of the function was integrated and two equations were obtained. These equations were solved by using genetic algorithm. Genetic algorithm showed that this method is a powerful tool to evaluate and optimize the rock mass geomechanical parameters. Moreover, the support system of the tunnel has been analyzed in this research. The support system of the tunnel includes implementation and installation of steel frames, wire mesh and shotcrete. The ground- support interaction method was used to determine the applied load on the support system. Capacity diagrams of the support system were plotted to evaluate the safety factor. The results showed that the support system doesn’t have enough safety.