Abstract: The variable diameter stabilizer is an important tool used in large extended reach drilling,and its functions,types and means of controlling were described in this paper.Taking several typical foreign products as examples,its working mechanism,typical structure and important specifications,as well as their applications and results in drilling tests and field operations were illustrated.A positive displacement variable diameter stabilizer was developed in China,and its field tests and real drilling applications in Well QHD32-6-A26 was presented.The results showed that this tool has stable and reliable performances.