The invention discloses a PBAT (polybutyrate adipate terephthalate) fully-biodegradable film composition and a preparation method thereof; film of the composition is not greater than 6 mm in thickness, greater than 40 MPa in transverse rupture strength, greater than 800% in elongation at transverse rupture, greater than 30 MPa in longitudinal rupture strength, and greater than 300% in elongation at longitudinal rupture; the magnitude of the rupture strength is not directional, and all transverse and longitudinal rupture strengths and elongations at transverse and longitudinal rupture are directional. The film acquired herein is fully biodegradable, has suitable service life, and is widely applicable to daily product packaging, express bags, heat-shrinking film, adhesive tapes, agricultural mulch and other aspects.