Risk assessment is essential to the safety of bridge construction.Ministry of Transport issued″highway bridge and tunnel construction safety risk assessment guidelines″（Trial）（hereafter referred to as″Guide″）in 2011.The″Guide″requires experts to use risk matrix based on cost level（C）and probability level（P）to estimate the level of risk.This method is easy to operate but has a low stability.Stability is defined as the fluctuating sensitivity of risk level with the change of assessment conditions.Risk assessment is a very subjective activity

fluctuations of C and Plevels will inevitably occur in the process of assessment

which leads to corresponding fluctuations of risk level.The improved method is based on the risk matrix in the″Guide″with a hypothesis that Cand Plevels have the same contribution to risk level.According to the principle of self-similar fractal geometry

a higher order risk assessment matrix is transformed from the basic one by refining the Cand Plevels.The refining process from 1to 0.1scale is realized by linear interpolation.The stability of risk assessment has enhanced observably through this improved method.