Abstract: The density value of film varied very much when testing small diameter tube by radiographic testing, which affected the reliability of defect detecting. For this reason, the radiographic testing parameters were analyzed for a  42 mm×5 mm 16MnR steel theoretically, then the sensitivity comparison testing were made on two kinds of situations with constant exposure and changing voltage, and also constant voltage and changing exposure . Testing results showed that, lower voltage and exposure would lead inadequate exposure on the film and cause lower density. But too high exposure would cause the reduction of image quality, so had not conducive to defect detection. Only when the exposure and tube voltage achieved an appropriate value, the good details of images could be obtained, and the radiographic testing effect could be assured.