Considering the effect of target period on reliability, the reliability adjustment coefficient γa was proposed in this paper, and the limit state criteria for safety evaluation of existing steel structure was established. By comparing different load reduction factor calculation method, the equal exceeding probability method was chosen and the load reduction factor of the different target periods of following service was obtained. Based on the checking point method, the reliability index of steel structure member with different target periods of following service was obtained. The result has shown that the proposed formula meet the requirements of target reliability index. According to the different satisfaction degrees of target reliability index, the classification criterion of the steel members was proposed and the limit value of parameter was given for the safe classes of structure and member importance.