When a hydroproject is being built at the downstream reach of a city, flood control becomes a very important task. Especially during the construction diversion, the width of overflow is reduced, which leads to water rising at the upstream. If the water rising is too high, the wealth and the life by the riverside can be seriously threatened. So this article aims at the study of the construction diversion schemes for the Changsha hydroproject by using 1：100 physical model test method. Based on the observed results of discharge capacity and waterpower characteristics, proposals on diversion′s spillway width and the plane arrangement types of the longitudinal cofferdam are optimized so as to effectively regulate the flow in the narrowed river. Thus the effective width of the overflow is increased, and the water rising and the upstream flooded area are reduced. The optimized construction diversion scheme has a marked flood discharge effect during Changsha hydroproject construction diversion, which can be for similar hydroprojects reference in future.