The invention discloses a construction method for quickly demounting anchored steel cofferdam by skillfully adopting water head difference in the sea. The construction method includes skillfully utilizing natural phenomenon of oceanic tide rising and falling, damaging binding force between the cofferdam and bottom sealing concrete by the pressure difference generated by water head difference inside and outside the cofferdam so as to separate the cofferdam from the bottom sealing concrete; pulling out and craning the whole cofferdam by a craning ship to the next pier for next construction. Therefore, the construction method has the advantages that construction procedures are simple and easy to operate; large jacking or underwater equipment is omitted, cost and investment are kept low; efficiency, quickness and safety are achieved at the same time; by the aid of natural resources, the construction method is energy saving and environment friendly.