Abstract ： Research purposes ： In recent years， the bridge collapse accidents have repeatedly happened. For fully recognizing the common mechanical mechanism and destruction causes behind of these accidents， so as to do well prevention and control work of security risk contrapuntally in bridge design，construction and maintenance stages，and to reduce accident rate of bridge collapse，the study was carried out. Research conclusions ：( 1) By analyzing many collapse accidents of bridges，the statistical characteristics of all kinds of bridges and ai kinds of destruction causes for bridge accidents were obtained. The results showed that the general safety of rigid frame bridge is the best and the arch bridges is the poorest correspondingly; the self - factors of construction is the main cause of bridge collapse in construction period，and the factors of vehicle and ship as vehicle overload is the main cause of bridge collapse in operation period. (2 ) By using mechanics metliod，the analysis and summary of common causes for collapse accidents were made on technological asj^ect. Then the cause of bridge collapse was put forward，and this is because that the external cause occupied the leading position in the contradiction of structural resistance and external force. (3)The prevention princijDle of security risl^ of bridges in design and construction stages is discussed and proposed, and the relevant suggestions in bridge design, construction and operation stages are also given. (4)The results can provide guidance and reference for the control work of security risk in bridge design ; construction and operation maintenance.