Abstract: During the long-term service of mechanical bimetal composite pipes, the liner stainless steel pipe will inevitably collapse, which can affect pipeline safety. Due to its structural characteristics and material properties, there are limitations in the detection of liner collapse by existing NDT methods. Proposed is a magnetic saturation pulsed eddy current testing (MSPECT) method with a novel signal processing algorithm. Magnetic saturation technology is used to overcome magnetic shielding effect of the outer layer of bimetal composite pipes, and enhance the depth of pulsed eddy current test (PECT). A signal processing algorithm is used to adaptively extract the "late correlation coefficient" as the feature for liner collapse detection, which ultimately realizes defect quantification. The principle of the proposed method is introduced and the feasibility of the MSPECT method in detecting the liner collapse of bimetal composite pipes is verified through finite element simulation. Based on this, a MSPECT experimental platform is built. The effectiveness and superiority of the method proposed are systematically verified through four types of experiments: comparative experiments, the validation of collapse detection using plate specimens, quantitative collapse degree detection using plate specimens, and quantitative collapse degree detection using pipe specimens. The method quantitatively characterizes liner collapse ranging from 0 mm~10 mm when the outer carbon steel pipe thickness is under 15 mm.