Abstract: After processed the signals obtained from monitoring system，one theory of condition evaluation was proposed on the basis of a new evaluation parameterindex variation． One new condition grade division method was proposed from approximate statistic distribution of index variation，and as per the concept of confidence degree and confidence interval． Because the monitoring data and the standard of condition grade division were all interval numbers，interval extension evaluation theory was adopted to establish the model of condition evaluation． Set-valued statistics and gravity center based decision theory were introduced to divide weight into subjective weight and objective weight to make calculation． Finally，the main bridge of Caiyuanba Yangtze River Bridge was taken for example to do evaluation and analysis by the monitoring data and the theory，and the result showed that the operation condition of main bridge of Caiyuanba Yangtze Ｒiver Bridge was good，and the result agreed well with the bridges actual condition，so this evaluation method was found to be reasonable and had good project practicability．