The invention discloses a multi-posture adaptable large-diameter pipeline inspection mechanism. The mechanism comprises a mechanical scanning part, an electric control part and a pipeline part, wherein the mechanical scanning part comprises a circumferential guide rail module, and the circumferential guide rail module comprises a plurality of fixed support legs and a plurality of retractable and adjustable support legs capable of rendering the inspection mechanism adjustable. Each adjustable support leg comprises a first support leg mounting base fixedly connected with a gear ring, multiple support leg guide rods with rear end parts penetrating through the first support leg mounting base, first fixing blocks fixed to the front end parts of the support leg guide rods, first buffer blocks fixedly connected to the first fixing blocks and retractable pieces mounted between the first support leg mounting base and the first fixing blocks. According to the multi-posture adaptable large-diameter pipeline inspection mechanism, the adjustable support legs in the gear rings are utilized to enable the inspection mechanism to be applicable to pipelines with different pipe diameters; an axial motion module can drive a probe to make adjustment at different angles and positions, and the comprehensiveness of scanning can be ensured through different inspection postures.