：Regarding the design and construction of the above-water and under-water structures of the concrete cofferdam，taking the longitudinal concrete cofferdam of the Yanzhou project phase I as an example，through the comparison and analysis of the structure design of the above-water part and the construction technology of the underwater part of the cofferdam，the gravity design scheme of the above-water part and the prefabricated casing technology of the underwater part are optimized.The results of theoretical calculation and field practice show that the gravity scheme has more advantages in reliability，structural stability and ease of construction.Compared with the traditional steel box technology，the prefabricated box technology can shorten the construction period by 22 days，avoid the steel box turnover，and save 1.83 million yuan construction cost.The research results can be used for reference in the design and construction of cofferdams of the same type of hydraulic engineering.