Abstract: In order to clarify the mistaken view that penetrant testing method is much more easier and were intuitive than magnetic particle testing for surface defect detection of vessel, so magnetic particle testing is not necessary to carry on. The principle, scope of application and respective advantages and disadvantages of the two testing methods were introduced. The conclusion was got that magnetic particle testing method could not only detect open surface defect but also near surface defect, which was verified at the sensitivity comparing testing for a floating head heat exchanger.