Abstract: Strictly following the theory of stochastic processes, it is indicated that the so-called dynamic reliability index used domestically is not the reliability index in the codes for the structural design. The former, an ambiguous concept, came from ignorance of abundant references of international pioneer studies and ignorance of the theory of stochastic processes. For the recent situation of safety problems induced by more and more aged bridges in China, the wrong concept misleads quite few researchers in their works on safety evaluation of aged bridges and leads to wrong conclusions of safety. Now is the time to clarify the wrong concert for avoiding its negative effects on aged bridge maintenance works.