The important status of guide ring in large power plants and its manufacturing methods were introduced. The advantages of manufacturing methods including welding and electrolysis were discussed.In order to ensure the brazing quality of the guide ring,ultrasonic nondestructive testing(UNDT) method was used in this study.According to the features of the defects that may show up during brazing,the feasibility and reliability of UNDT were proved in theory.Based on the theory,an ultrasonic C-scan imaging test method was developed using a water immersion focusing probe.According to the features of the inspected images,the defects were segmented using a morphology image processing based method.The defects can be localized and sized by employing the algorithm of area reconstruction.At last,the combination ratio of brazing interface can be calculated and the brazing quality of the guide ring can be evaluated. The important status of guide ring in large power plants and its manufacturing methods were introduced. The advantages of manufacturing methods including welding and electrolysis were discussed. In order to ensure the brazing quality of the guide ring, ultrasonic nondestructive testing (UNDT) method was used in this study. According to the features of the defects that may show up during brazing, the feasibility and reliability of UNDT were proven in theory. based on the theory, an ultrasonic C-scan imaging test method was developed using a water immersion focusing probe . According to the features of the inspected images, the defects were segmented using a morphology image processing based method. Defects may be localized and sized by employing the algorithm of area reconstruction. At last, the combination ratio of brazing interface can be calculated and the brazing quality of the guide ring can be evaluated