Abstract: The detection principle of alternating current field measurement (ACFM) technique is described. The probabilities of detection of ACFM and MT methods are analyzed and compared, according to many specimens with different crack dimensions machined in the lab. Considering the structural characteristics and the special working environment of offshore platform, the software and hardware system which can display the realtime audio and video and the ACFM inspection results synchronously during testing is proposed. In the meanwhile, the point estimate value and confidence lower bound not based on prior information by virtue of the Bayes theorem are obtained respectively, which can almost accurately evaluate the crack existing probability. The application of the presented technique and approach by a jackup platform proves that the outcome is good, having the value of practical application.