The invention discloses flat-section cellulose diacetate fiber and a preparation method thereof. The preparation method includes that cellulose diacetate fiber powder and polyoxyethylene are dissolvedinto a mixed solvent of acetone, water and alcohol to prepare a spinning solution; an electrostatic spinning process is adopted, drops of the cellulose diacetate spinning solution fall apart into tiny drops under action of electrostatic high-voltage electric field force, and the tiny drops are drawn to be micron-scale fiber with flat section to form a fiber film on a receiver. Compared with the prior art for preparing flat fiber, the preparation method has the advantages of being simple in operation and easy-to-control in condition. The film composed of flat-section cellulose diacetate fiberhas the advantages of large specific surface area, high dust containing capacity, high hydroscopicity and excellent adsorption performance and has good application prospect in the field of tissue scaffolds and filtering materials.