The invention provides a small-bore bidirectional two-dimensional floating self-adaptive mechanism for a high-temperature and high-pressure underground pipeline. The small-bore bidirectional two-dimensional floating self-adaptive mechanism is characterized in that detection arm components are mounted around a center framework of the self-adaptive mechanism, two sensor seats are mounted at two ends of the center framework respectively, and displacement sensors are evenly mounted around each sensor seat; the detection arm components are mounted on the center framework and are adaptive to the inner diameter changes of a measured pipeline to allow probe modules to constantly fit the inner wall of the pipeline; displacement sensor probes are connected with probe push rods to convert the inner diameter radial changes of the pipeline into axial displacement of the displacement sensors. The small-bore bidirectional two-dimensional floating self-adaptive mechanism has the advantages that peripheral 360-degree inner diameter variation and defect detection of the pipeline can be realized, and each group of magnetic circuit modules can radially float independently to adapt to pipeline inner diameter variation conditions; the mechanism can axially bidirectionally move to adapt to the bidirectional movement of descending and retracting of a weak magnetic well diameter combined sleeve logging instrument in the pipeline, the problem of detector passability is solved, and whole product research and development are assisted.