The invention discloses a preparation method of a milk protein fiber and cotton blended fabric and belongs to the field of spun-dyed fabrics. The preparation method of the milk protein fiber and cotton blended fabric, disclosed by the invention, comprises the following steps: pre-treating cotton fibers through a sodium hydroxide solution, washing with water until the cotton fibers are neutral and drying; pre-treating milk protein fibers through a weak acidic dyeing prevention agent and drying; carrying out scutching and cotton carding procedures on the pre-treated cotton fibers and the pre-treated milk protein fibers, blending and drawing; treating by a roving frame and a spinning frame to obtain the milk protein fiber and cotton blended fabric; putting the milk protein fiber and cotton blended fabric, a neutral dyestuff, a dyeing promoter and a curing agent into a constant-temperature water bath pot containing water, and dyeing and fixing color; then washing with the water, soaping and drying to obtain the dyed milk protein fiber and cotton blended fabric. By adopting the preparation method, the union dyeing property of the blended fabric is improved.