The invention relates to a preparation method of a high strength polyamide 6 fiber. The preparation method comprises steps of preparation of a chain extender, preparation of modified hectorite, preparation of acid/anhydride grafted modified hectorite, preparation of high strength PA6 composite resin, and preparation of high strength PA6 composite fiber. Hectorite is modified by KH570, then modified hectorite reacts with unsaturated dibasic acid or dibasic anhydride, and thus the compatibility between hectorite and a polymer matrix is enhanced. Amino groups are introduced into hectorite, the action force between hectorite and PA6 molecular chains is enhanced, the orientation of hectorite easily changes along with the orientation of the PA6 molecular chains; the crystallization performance of the composite resin is enhanced, the mechanical properties of the composite resin are improved; the mechanical properties of prepared high strength PA6 fiber are effectively strengthened; and the prepared PA6 fiber can be applied to fields such as textiles, automobile products, internal ornaments of airplanes and high-speed rails, military clothes, and the like.