The invention relates to a preparation method of boron nitride/sialon composite material by adopting boron-rich slag. The preparation method comprises two steps: the first step is that synthesis sialon powder BN/ (Ca, Mg) Alpha' : 1. crushing, 2. ball milling, 3. screening, 4. magnetic separating, 5. proportioning according to mass percent: 7.94 to 45.79 portions of boron-rich slag, 12.19 to 58.78 portions of silicon fume, 3.82 to 19.78 portions of bauxite, 21.70 to 35.41 portions of carbon black, 6. wet mixing with anhydrous ethanol as the medium, 7. drying with the temperature 60 DEG C; 8. dry mixing, 9. molding forming, 10. high-temperature sintering under one atmospheric pressure, temperature of 1450 to 1500 DEG C, insulation for 6 to 10 hours and protection of nitrogen; 11. eliminating carbon residual by burning; the second step is that mixing the powder obtained by the first step and additive CaCO3, and after the molding forming, the final product sialon ceramic composite material BN/(Ca, Mg) Alpha' is made by sintering through burying powder in furnace under temperature 1600 to 1700 DEG C; all properties of the product are good. The invention has simple process, low production cost, and starts a new way for boron-rich slag integrating utilization and decreases environment pollution.