The invention relates to a flame-retarding polyamide resin composition and a preparation method thereof. The preparation method comprises the following step: melting and blending polyamide resin, a tin-containing compound, organic hypophosphite and a processing auxiliary agent to prepare the flame-retarding polyamide resin composition. The tin-containing compound is more than one of tin salt I, tin salt II, tin salt III, a complex and hybridized salt; the tin salt I, the tin salt II and the tin salt III are compounds formed by combining metal ions and acid group anions through ion bonds; the metal ions are tin ions or stannous ions; the acid group anions are cyanurate ions, hypophosphite ions and 2-carboxethylphenyl hypophosphite ions; the complex is a compound formed by combining triphenylphosphine and the tin salt I or thin salt IV through a coordination bond; the tin salt IV is stannous chloride or tin chloride; the hybridized salt is melamine cyanurate formed by hybridizing tin salt I, the tin salt II, the tin salt III or the complex. The method is simple and feasible and the prepared polyamide resin composition has a good flame-retarding effect.