Shandong Qufu Xingtan Theater, originally built for an open-air theater, was designed as a one-time temporary structure with bolt ball junction double-layer regular pyramids grid structure roofing system. Because of the transformation of open-air structure system to an enclosed construction, some members of the space grid structure can not meet the required bearing capacity. According to the mechanical performance of space grid structures, the reinforcement methods of “circular steel jacketing” and “lateral support” are conducted, which are based on the analysis of the stress and stability characteristics of different grid members. By using the methods given above, the stress and slenderness ratio is decreased. The results indicate that the two reinforcement methods can reduce the difficulty in construction, shorten the construction period, and gain good economic and social benefit.