The invention relates to a fast island-constructing caisson. The fast island-constructing caisson comprises a prefabricated caisson, a protruding part is arranged at the bottom edge at one end of the caisson, and a matched recess part is arranged in the corresponding position of the other end of the caisson. A construction method includes the following steps that when the caisson is loaded to sink, the end, provided with the recess part, of the caisson is aligned with the protruding part of the prior caisson which has been installed already, the recess part of the next caisson is placed on the protruding part of the prior caisson, and the other end of the next caisson is supported by a supporting device. The fast island-constructing caisson is good in stability, construction is facilitated, an island can be constructed fast, construction speed is high, consumed materials are saved, the construction period is short, the influences on the surroundings are very little, the cofferdam dismantling workload is small, and the engineering building cost is low.