The invention provides a preparation method of a composite material. The method comprises the following steps: crushing a bacterial cellulose membrane through a high-pressure mechanical homogenizing technique; preparing homogenized bacterial cellulose, and carrying out hydrolytic-acidification reaction; washing after acidification treatment, and then centrifuging, so as to obtain microcrystalline cellulose; and dispersing the obtained microcrystalline cellulose in graphene oxide dispersion liquid. The composite material is as shown in a figure 1. The composite material prepared by the invention has relatively good antibacterial property.