Abstract: Analyzed here in the article are the causes for the quality-affecting pit defect of the hot-rolled seamless steel tube. As a result it is identified that the said pit defect is formed on the one hand，due to the borax accumination that is caused by poor borax powder dispersivity of the pierced shell inner surface borax operation，and on the other hand，owing to the deviation of the MPM-the extracting mill center line that leads to scratching and hitting the extractor by the pipe end，and getting a small amount of the metal peel and fall into the frame in front of the extractor to make the small pit defects formed on the pipe external surface. Accordingly relevant counter measures are taken，including optimizing the borax powder components and the sprayer structure，periodical calibrating the MPM-the extractor rolling center line with the on-line center calibration system，and optimizing the extractor frame structure，etc. As a result any fallen metal particle has no longer been found inside the extractor frame，and thus the pipe pit defect reject rate has been reduced to 0.05%，which means that the pit defect has been basically eliminated.