Abstract: The risk of failure of improving the existing dam safety during construction must be considered and weighed against the benefit obtained from the modification,and what can be done to minimize the risk during construction should be taken into account.In Tsengwen Dam Raise Reservoir Water Level Project at Taiwan,the greatest construction risk is the overtopping of the dam by flood when the crest is lowered by excavations.To secure the dam safety during construction,based on seepage analysis,required freeboard elevation check,stability and dynamic stress analysis of the dam,and flood routing,and considering the balance between optimum performance of the existing dam and economy and construction schedule,a compromise and integrated design should be needed instead of the mere construction of either a new upstream or a downstream filter zone.