In the dock expansion construction on an island，the cofferdam provides a dry working environment for dock construction. Cofferdam construction is a key link in dock expansion construction and determines its success or failure. Taking into account engineering practice and local conditions，this study optimizes the form of the temporary cofferdam structure for dock expansion under the complex hydrogeological conditions on the island. After developing a new type of composite cofferdam structure，this study verifies that it meets stability and impermeability requirements through finite-element modeling and checking and that it offers a favorable dry working environment for dock expansion construction. Practice proves that the proposed cofferdam construction technology for dock expansion achieves remarkable results.