Abstract: For the in-service inspection of marine oil electrostatic dehydration pressure vessel, ultrasonic guided wave method was used. For the inside and outside wall corrosion defects of this type of containers, monoclinic ultrasonic guided wave transducer was developed, and experiment and site test were done in the research process.Test results showed that the  6 mm×1.25 flat-bottom hole of the 1 400 mm×600 mm×5 mm sample could be clearly detected by the use of self-developed guided wave transducer, and the detection range of the length being up to 1 000 mm. It was able to detect the inside and outside wall corrosion defects of marine oil electrostatic dehydration pressure vessel. Due to the surface paint layer and the internal liquid medium, the acoustic attenuation was large. In order to achieve the desired effect, a detection range length should not be more than 500mm. Test results show that the ultrasonic guided wave detection method can realize rapid detection of corrosion defects of the in service pressure vessels.