The safety joint is a kind of RH safety joint that used in DST, it connect to the upper end of packer to keep the tool left in hole is the list while need decoupling the safety joint if jam releasing failed, but the tools and drill string on the upper side of safety joint still can obtain formation data. Then release jam by adopting other methods, and thread safety joint after releasing jam to tripping out the lower drilling string. The safety joint also can be used in various fishing work, drilling and workover operation.1. Structure character and working principle