The invention discloses an expansion method for an existing subway underground excavation overlapping island transfer station. Expansion can be conducted by constructing an external-hanging open excavation platform structure on one side of an existing transfer station, and the newly-established external-hanging open excavation platform structure and the existing transfer station are connected through an underground excavation connecting channel constructing from one side of the open excavation structure. According to the expansion method for the existing subway underground excavation overlapping island transfer station, by newly-establishing the open excavation platform structure, the width of the station platform doubled; transfer passengers in the existing overlapping island station halved, and thus the transfer ability of the existing station is improved in a multi-fold mode. According to the expansion method for the existing subway underground excavation overlapping island transferstation, the newly-established connecting channel is constructed by using an underground excavation method, the engineering occupation land is reduced, and influence on road traffic is avoided. According to the expansion method for the existing subway underground excavation overlapping island transfer station, construction such as construction of the newly-established channel and existing stationport, diversion of existing cables and transformation of station gates and signal systems are all conducted in skylight time to avoid the influence on operation of stations.