The invention provides a preparation method of a composite material for polypropylene modification. The preparation method comprises: blending wollastonite powder by using water to obtain a 50-60% slurry, and adding sodium stearate; preparing a 65-70% composite slurry from barium sulfate and titanium white powder according to a mass ratio of (4-6):1 by using water, and adding sodium stearate and a silane coupling agent; carrying out wet grinding on the prepared titanium white powder-barium sulfate composite slurry to 3000-4000 mesh; adding the prepared wollastonite slurry to the prepared titanium white powder-barium sulfate composite slurry according to a mass ratio of the titanium white powder-barium sulfate to the wollastonite of (10-12):1, and adding a silane coupling agent to coat; drying the composite slurry to obtain composite material dry powder; and beating and disaggregating the composite material dry powder to achieve the dispersed state, carrying out a cyclone grading treatment, and collecting the 2500-3000 mesh powder. According to the present invention, the composite material product with characteristics of high weather resistance, high gloss and high chemical stability can be obtained.