The work aims to introduce poly ( N-diol fumarate) (PNF) into PLLA as a flexible chain segment by melt copolymerization to complete the modification, so as to improve the flexibility and gas selective permeability of PLLA. A series of poly (L-lactic acid-diol fumaric) (PLNF) copolymers were prepared with PLLA as the main component by adding PNF and changing the length of the diol chain segments therein, which were subsequently prepared into films, tested for mechanical properties, gas permeability, and water vapor permeability, and subject to storage tests on portobello Tricholoma mongolicum Imai. The mechanical properties, gas selective permeability, and moisture permeability of PLLA were improved by the addition of PNF. Compared with PLLA, the elongation at break of poly (L-lactic acid-decanediol fumaric) (PLDF) could reach 346.4%, the selective permeability ratio of CO2/O2 of poly (L-lactic acid-butyrate fumaric) (PLBF) increased from 3.0 to 4.4. During the whole storage process, the Tricholoma mongolicum Imai. could maintain morphology, present better sensory quality, and still have edible value within 15 d. The addition of PNF effectively improves the properties of PLLA and increases the shelf life of Tricholoma mongolicum Imai., which provides a reference for the further use of PLLA films for fresh fruit and vegetable packaging.