The capacity of existing RC bridge could decay slowly when exposed to the aggressive environments. In this paper, the probability assessment method was presented. And the probability model of dead effect and vehicle action effect was established. The classification principle of bearing capacity was presented on the basis of reliability index; furthermore, the evaluation method was presented by a lot of calculation about the reliability index and the ratio of vehicle action effect to dead effect. An existing RC beam bridge with 34 years was evaluated; the technology condition is third class according to code for maintenance of highway bridges and culvers (JTG H11-2004). The checking index is 0.947 and the assessment result is also third grade. Comparing of the results indicated that the method is practical and convenient.