Load effects and resistance are supposed to constant in traditional reliability theory. In fact, load effects and resistances change with time because some influent factors of structures change in long working life. Thus, only structures are safe at every moment in working life are structures safe. A synthetical method of qualitative and quantitative analysis is proposed in this paper to study the time-dependence problem of a RC free beam. Firstly, the time-dependence character of a free beam is simulated by ANSYS program. Then, a time-dependent reliability model of the free beam is established, and a program is edited to get values of resistance and index of reliability in different years, thus, the time-dependent reliability of the RC free beam is assessed. The results showed that the value of the resistance decreased 20 percent and the reliability index cut half down after 50 years, so time-dependence should be considered to achieve correct results when the reliability of in-service structures is estimated and new structures are designed.