Abstract: In order to ensure the needs of production and life, LNG (liquefied natural gas) pipelines need to be in-service inspected of the remaining wall thickness of the pipeline without removing the cold insulation layer. Taking the LNG cold preservation pipeline as an example, the article analyzes the basic principles of tangential ray detection technology, and uses the developed tangential ray detection test device to test a section of LNG pipeline, and then compares its detection accuracy with the micrometer measurement results. Following the analysis, the remaining wall thickness of an in-service LNG pipeline was tested and combined with the Gumbel extreme value distribution method to predict the minimum remaining wall thickness of the entire LNG pipeline. The test results show that the tangential ray detection method can be effectively applied to the wall thickness detection of LNG pipelines with cold insulation and can predict the minimum remaining wall of the entire LNG pipeline, and the minimum remaining wall thickness of the LNG pipeline conforms to the Gumbel extreme value distribution.