In order to investigate the emergency capacity of the full prestress and A-class components under normal service conditions, vertical tensioned increment method was be applied to study the statistic discipline of effective prestress for existing PC bridges. The results showed that the statistical parameters of effective prestress obey normal distribution and ~N(0.97,0.09). On this basis, the stress-resisting probabilistic models of two kinds of components were constructed. The dead load effect probabilistic model and the vehicle load probabilistic model considering evaluation base period were established. Based on the research above, the assessment method of stress reliability for PC bridge under normal service conditions was presented.