The invention relates to a preparation method of a micro-nanocellulose modified wood-plastic composite (WPC), and belongs to the field of high polymer materials. According to the preparation method ofthe micro-nanocellulose modified WPC, a biomass material or cellulose is used as a raw material to efficiently prepare micro-nanocellulose through a swelling-phosphoric acid dissolution-ball millingmethod. The micro-nanocellulose is added into WPC to improve comprehensive application performances of the WPC. The preparation method of the WPC comprises the following specific steps: 1, preparing apremix 1 of the micro-nanocellulose and plant fibers; 2, mixing the premix 1 with other components in the formula of WPC to obtain a premix 2; 3, granulating and molding the premix 2 to obtain the micro-nanocellulose modified WPC. The problem of dispersion of the micro-nanocellulose in WPC is solved by adopting a simple and ingenious method, and the purpose of improving comprehensive performancesof the WPC is achieved. The preparation method of the micro-nanocellulose modified WPC disclosed by the invention is environmentally-friendly, does not require any chemical reagents and special equipment, and is a method suitable for industrial mass production.