The invention discloses a method for producing high-strength high-tenacity polypropylene fibre. The method is characterized in that the former spinning draft and latter spinning draft double-twisting two-step method is adopted; the strength of the produced high-tenacity polypropylene fibre can be higher than 8.5g/D; a large-draw-ratio screw extruder with draw ratio larger than 28:1 is used to improve the mixing and homogenizing effect of a polymer melt; an ultralarge-draw-ratio spinning assembly spinneret orifice with draw ratio larger than 4:1 is adopted to stabilize melt flow, reduce the swell ratio of a melt outlet and improve the uniform tenacity of the fibre; and the polypropylene powder with low price is used and 3-5% of high-density polyethylene and 2-4% of polypropylene cooling master batch are added. The method has the following beneficial effects: the produced high-tenacity polypropylene fibers have high strength; the process is simple; the operation window is wide; and the method is economical and practical.