Selective transformation of carbon dioxide and epoxides into degradable polycarbonates (CO2-based copolymer) has been regarded as a most promising green polymerization process. Although tremendous progress has been made during the past decade, very few successful examples have been reported to synthesize well-defined block copolymers to expand the scope of these green copolymers. Herein, we report a tandem strategy combining two living polymerization techniques, salenCo(III)X-catalyzed styrene oxide SO/CO2 copolymerization and ring-opening polymerization of lactide with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), for the synthesis of poly(styrene carbonate-block-lactide) copolymers. The key to the success of this tandem strategy is the judicious choice of water as the chain transfer and/or chain terminator reagent, which is added at the end of the salenCo(III)X-catalyzed SO/CO2 copolymerization to in situ generate hydroxyl groups at the end of the polymer chains. The resulting polycarbonates with -OH end groups can thus be directly used as macroinitiators to subsequently initiate ring opening polymerization of lac-fide to synthesize the diblock copolymers. Because of the living polymerization nature of both steps in this tandem strategy, we have demonstrated that the diblock copolymers synthesized possess well-defined structures with narrow molecular weight distributions and controllable lengths of both styrene carbonate and lactide blocks.