A method for preparing stably dispersed cellulose nanofibers comprises the following steps: 1) mixing cellulose and an organic solvent, the percentage of the cellulose being 1% to 15% in weight; 2) adding an esterification agent into the resultant mixture of step 1), the molar ratio of the esterification agent to the cellulose being from 1:0.1 to 4; and 3) physically breaking the resultant mixture of step 2) until a suspension liquid with stably dispersed cellulose nanofibers of 2-1000 nm in diameter and 10-100 μm in length is obtained, an esterification reaction of hydroxyl group(s) on the surface of cellulose fibers occurring at the time of the breaking. Also disclosed are dispersed cellulose nanofibers with improved compatibility to the matrix than the untreated cellulose and an improved strength of the composite materials.