The invention provides a water foundation construction method. The method comprises the following steps that firstly, a cofferdam is constructed around a bearing platform, the cofferdam is filled withsand-gravel materials, and an artificial island is formed; a steel casing is constructed on the soil island in the cofferdam to the appropriate depth; mud jacking is carried out below the bottom of the bearing platform in the cofferdam at the appropriate depth to from a stably sealed layer; a drilling pile is constructed on the artificial island according to land construction requirements; the sand-gravel materials in the cofferdam are excavated till reaching the bottom of the bearing platform; and the bottom of the cofferdam is cleaned, bearing platform construction is carried out, and foundation construction is completed. After the drilling pile is changed from water construction into land construction, large equipment which is more efficient can be used, the construction efficiency isimproved, construction quality and safety are more easily controlled, and construction can be completed safely, rapidly and efficiently with high quality. In addition, a construction steel platform does not need to be erected during drilling pile construction, the steel consumption is reduced, and the construction cost is effectively reduced.