To study the mechanical properties of spatial thin-walled elastic extension bar, the change of energy, external load during leveling and flattening and stiffness are analyzed. Firstly, the stiffness of elastic extension bar is theoretically analyzed, and the parameters that affect the stiffness of elastic extension bar are found out. Then, the process of flattening and leveling the elastic extension bar is simulated by ABAQUS, and the influence of various parameters on the stiffness is analyzed by using the control variable method. The simulation results show that the energy variation in the leveling process is basically consistent with that in the flattening process, but the external load required for the flattening is relatively small. With the increase of the radius of arc section, the number of layers and the center distance, the deformation of the elastic extension bar will gradually decrease under the same load. By the analysis of the simulation results, it can be seen that the external load is smaller in the initial stage of flattening and leveling, and the peak value of the external load appears in the final stage of flattening and leveling, and the stiffness of elastic extension bar can be improved effectively by increasing the radius, center distance and the number of layers.