The invention discloses a horizontal well gravel packing tool which comprises a connecting collar, an outer cylinder, a liner, an upper core tube, a core tube collar, a lower core tube, an outer cylinder centralizer, a spring sleeve, a sliding sleeve, a spring, a liner guider, a wire-wrapped screen, a plug and a guide head. The tool is capable of conveniently and mechanically positioning, heavy work of largely and accurately calculating tubular columns is decreased for constructors, a central inserting tube is only tripped in, fixed and lifted out, simple to operate, safe and reliable during construction, a packing port of an outer tool is closed twice by the aid of the tool, gravel regurgitation is avoided after construction, and a packing port of the central inserting tube falls by a certain depth and enters the bottom of an inner cavity of the outer packing tool after the central inserting tube is positioned. By the aid of the structure, gravels cannot enter the space between the central inserting tube and the outer packing tool even if the tubular columns move when the central inserting tube is packed, so that the major accident of sand jamming of the tubular columns is avoided.