The invention relates to a modified polypropylene fiber and a preparation method thereof. The invention is characterized in that the fiber has a skin-core structure, wherein the core layer is composed of pure polypropylene; and the skin layer is composed of 0.5-60 parts of functional material, 0-5 parts of compatilizer and the balance of polypropylene, totaling 100 parts. The viscosity-average molecular weight of the polypropylene is 0.18-0.3 million, and the degree of isotacticity is greater than 95%. The functional material is a functional polypropylene fiber master batch prepared by blending and granulating inorganic or organic functional micropowder, a compatilizer and polypropylene, wherein the functional micropowder is composed of one or more of nano silicon dioxide, nano titanium dioxide, nano silicon dioxide, an anti-mite agent and an N-substituted alkoxy hindered amine flame retardant. The polypropylene fiber can be subjected to skin-core conjugate spinning according to the skin-core formula, and can be further processed into a polypropylene fiber spunbonded nonwoven fabric or melt-blown nonwoven fabric by spunbonding or melt-blowing. The method has the advantages of simple production technique and small modification for equipment, and is easy for practical popularization and application.