Abstract: As ordinary packer is apt to set position halfway in the erosion process, the fluid cylinder is prone to get sand in due to incompletely sealing and the sand screen in horizontal well can’t run down while washing. Therefore, the undershoot packer in horizontal well is developed to solve the issues. The packer is optimized from structure in which the design of collapsible setting ensures the safe and reliable setting（sealing pressure can reach to 30 MPa）and the sealing arrangement with rubber contracting to protect is to guarantee completely sealing. The indoor test and field application indicate that the device features high innovation, safe and reliable comprehensive performance, and it can perfectly meet the requirement of well completion and production in the casing of 177.8 mm/139.7 mm.