The invention relates to a preparation method of a composite material with the fiber surface coated with cellulose nanocrystals and used for multi-scale enhancement. By adoption of the preparation method, the technical problems of poor macro-mechanical properties, low preparation efficiency and high cost of an existing fiber composite material are solved. The preparation method comprises the following steps: (1) putting the cellulose nanocrystals and deionized water in a device for mechanical stirring, carrying out ultrasonic dispersion and obtaining a sizing solution; (2) fixing unsized continuous fiber on a traction machine, and removing water by an impregnating vessel filled with the sizing solution and then winding for standby application; (3) preparing the obtained continuous fiber with the surface coated with the cellulose nanocrystals into a composite material strip sample, and curing according to the curing procedures of a selected resin system and obtaining the fiber-enhanced composite material coated with the cellulose nanocrystals. The preparation method can be widely used in the field of preparation of the composite material.