The invention relates to para-aramid fine denier fiber and a preparation method thereof. The method comprises the following steps: dissolving a para-aramid polymer in a strong acid, filtering twice, defoaming, performing tertiary filtration, spinning, stretching, cooling to solidify, rinsing, drying and rolling to finally prepare the finished fiber. In the preparation method, a high shear double screw extruder and a continuous film defoaming device are adopted, multiple filtering working procedures are adopted, the filtering precision characterized by the filtering plugging value Kw which is no more than 150, the defoaming purity, the spinning temperature, the stretching condition and the like are strictly controlled, the global optimization of process steps and process conditions is performed, the method is practical and is high in operability, and the para-aramid fine denier fiber with good physical properties can be prepared. The para-aramid fine denier fiber has the advantages of high strength and good quality, the fineness is 30-100D, the tensile strength is no less than 22CN/dtex, the elongation at break is no less than 2.5% and the initial modulus is no less than 550CN/dtex.