Abstract：There are a lot of cultural relics which are of important historic significance and value in China. But，because of long time weathering，these cultural relics are damaged seriously. It is very necessary to assess the weathering degrees of these relics. Ultrasonic detecting is a non-destructive testing method to get the inner information of an object with high resolution，so it is an effective means to detect weathering degrees of lithoid cultural relics. The ultrasonic CT detecting method for weathering degrees of lithoid cultural relics with ultrasonic penetrating waves is discussed theoretically and methodologically，and a set of methods to detect the weathering degrees and the main mechanical parameters distribution of lithoid relics are put forward. By this method，the weathering degrees of the stone bars of Guyue bridge，which was built in Nan Song Dynasty in Yiwu City，Zhejiang Province，are detected，and satisfactory results are obtained.
