The invention relates to a preparation method of a para-aramid low-denier fiber filament, belongs to the technical field of chemical fiber spinning, and particularly relates to a preparation method of a para-aramid low-denier (25-100D) fiber filament. The problem of wire breaking needs to be solved in the spinning process of the para-aramid low-denier fiber filament to ensure the production stability and the tow performance stability. According to the preparation method of the para-aramid low-denier fiber filament, a selected spinneret cap is used for reducing the risk of extruded cross flow and doubling of a para spinning solution; an alkali liquor with proper concentration is used for washing primary tows, thereby quickly neutralizing sulfuric acid and improving the strength of the tows; and by adopting flat washing and drying modes, the risk of wire breaking in the washing and drying links is reduced. By using the method, the spinning process of the para-aramid low-denier filament is stable and easy to operate, and the obtained fiber is excellent in mechanical property and stable in quality.