Abstract: Experimental studies were conducted to S30432 high-temperature superheater tubes with relatively high magnetism respectively using magnetic induction intensity measuring instrument, austenite oxide detector, optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffractometer, etc. Results show that new tubes have no magnetic equivalent and magnetic induction intensity; higher magnetic equivalent and magnetic induction intensity exist in tubes after service for 1.9×105 h, both of which change greatly in the annular direction, the longer the service time, the higher the magnetic equivalent; causes leading to high magnetism of the tubes are the existence of lath martensite, distributed closely at both sides of the grain boundary; the amount of martensite structure in tubes with longer service time is significantly larger than that with shorter service time; compared with new tubes, the ε-Cu phase dispersed intragranularly in S30432 service tubes is obviously greater in size, which turns to round particle from its original irregular shape.