In order to hurdle the limitations of Monte Carlo method and Response Surface method, the advanced Monte Carlo method was shown in this paper. And the system reliability of a one-bay and three-story steel framework structure was analyzed using the improved Monte Carlo method. Results of reliability analysis showed that there were high precision and a short calculating time using the improved Monte Carlo method. Then, the model of optimal design for steel frame based on the system reliability was built in accordance with the improved Monte Carlo method. And the optimal design on the steel frame was shown using the modern optimization method--genetic algorithm. From results, it was proved that optimal design without reliability could make structure have not enough reliability, even would not reach the required level of structural safety of Chinese criterions. It may lead to hidden dangers when only to reach objection on economy but haven’t considering reliability.