Abstract: To solve the problem of extension of casing string in the horizontal well in thermal recovery development,a kind of prestressing tool in horizontal well screen completion was developed,which could relieve casing damage and wellhead lifting problem by the method of prestressing casing string on the upper part of horizontal section.Laboratory and field tests indicated the prestressing tool could prop up casing string off the ground completely and ate into the formation obviously,it generated pull lift force effectively and could be up to 100t safely,the prestressing tool could meet the requirements of the horizontal well screen completion,fill the gaps of prestressing tools in the horizontal well completion and it was worthy of being used widely.