A horizontal well completion tubing string capable of achieving sublevel mining comprises a production casing. A horizontal section of the production casing is connected to two primary screen pipes. A pumping pipe is fixed in a vertical section of the production casing via a packer. A horizontal section of the pumping pipe extends freely in a horizontal well section constituted of the two primary screen pipes and the casing. A secondary screen pipe is disposed on the horizontal section of the pumping pipe. Meshes of the secondary screen pipe each comprises a reduced section, a straight section and an impact section, wherein an inner diameter of the reduced section is reduced outwardly along the radial direction of the secondary screen pipe, and an inner diameter of the impact section is reduced outwardly along the radial direction of the secondary screen pipe.