Abstract: The use of the gas-injection underbalanced drilling(UBD) in horizontal wells or high-angle deviated wells is one of the significant aspects to broaden the UBD application. Currently, gas-injection measures include mainly the drill-string gas injection and annular gas injection. When the drill-string gas-injection measure is used, the conventional downhole dynadrill and mud-pulse measurement-while-drilling(MWD) tools usually can not work normally, while existing annular gas-injection measures, such as parasitic casing and dial casing, are not applied widely due to higher cost and many technical problems. Therefore, combined with the theory and practice of underbalanced drilling and horizontal wells, the present paper proposed a new annular gas-injection measure with attached casing, and its key techniques, including the design of advanced gas-injection casing head, the proper size design of casing and bit series, and the depth calculation of the annular gas-injection point, were theoretically analyzed and verified. This gas-injection measure solved some special problems in drill-string gas injection, constituting a set of feasible annular gas-injection underbalanced drilling techniques for horizontal wells.