Abstract: For solving the problems faced by manual testing of large tapered-cylinder forgings, the automatic ultrasonic C scan imaging detection system with 5 degrees of freedom was designed by studying DAC fitting method and pseudo color imaging principle. Test data are represented by pseudo color image. One large tapered-cylinder forgings containing defects was tested. Result showed that the system could replace manual testing to achieve automatic detection of large tapered-cylindrical forgings and the detection result was more intuitive and reliable, thus making the quantization and location of defects become more simple and fast. Therefore the detection efficiency was improved and labor intensity was reduced.