Back-off sub

The utility model provides a novel back off sub corollary to an oil field completion string, and is applicable for large pitch directional wells and horizontal wells. The utility model comprises an upper joint (1), a lower joint (2), a sliding bush core (3), a lock ball (4) and a cutting nail (5). The utility model is characterized in that the upper joint (1) is connected with the lower joint (2); the sliding bush core (3) is arranged in the upper joint (1); the lock ball (4) is arranged between the lower joint (2) and the sliding bush core (3); the lower joint (2) is connected with the sliding bush core (3) through the cutting nail (5). By adopting the novel back off sub, bowling or pressurization is not adopted for releasing. When the utility model is used in large pitch directional wells and horizontal wells, the problem that iron shots can not be put in place for pressurization and sealing fails is prevented. In addition, a dead water pressure relief passage is designed, the defect that water in dead wate