Abstract: To detect the reasons for the damaged steel structure,and find the processing method and advices effectively,a method based on the improved case-based reasoning(CBR) is presented for diagnosing the damaged reason.This paper describes the analytic model of the damaged characteristics and reasons in detail,and puts forward the method expressing the analytic results using the standard damaged characteristic and reason relationship matrix.The matrix parameter is a probable index,expressing the relationship between characteristics and reasons.Then,all cases are concluded to obtain the empirical matrix,and the main reasons for the accident based on the characteristics of the actual accident.This method has been compiled to the artificial intelligence software,using as a convenient and effective aid for the engineering accident staff to analyze.