Abstract: In view of the high casing deformation rate as occurring during the operation of the shale gas horizontal well,a dual-layer string design is proposed.It is identified via relevant study that the dual-layer string service can be realized by means of running twice and staged cementing;the inner casing string is hanged near the A point,and the clearance between the two casing layers meets the need of cementing;the centrality of the inner casing affects obviously the collapse strength of the two-layer casing,while the cementing quality affects remarkably the collapse strength of the two-layer string;and it is emphasized that in case of uniform load and good cementing quality,the collapse strength value of the two-layer string is far beyond the sum of the collapse strength values of both the inner and outer casings;and the approximate collapse strength value of the two-layer string can be calculated by relevant theoretical formula.The above mentioned research achievement is worth using as the reference for the design of well depth structure,and the calculation of collapse resistance value of the two-layer string.