For a long time, the allowable value of deflection in the middle span of girder bridge under constant load [ft] has been vacant in Chinese bridge regulations. Generally, live load deflection fp=[L/600] is used instead. After nearly 20 years of research, it is considered that this is an important reason for the common downwarping and floor cracking of P.C continuous beams in China. Through the analysis of the paper, it is proposed that the design should control the initial deflection before closing [f]≤L/4800 to ensure the safety of the bridge.