The solution casting method was used to prepare polylactic acid/modified nanocrystalline cellulose (PLA/mNCC) composites, and the effects of the mNCC content on the morphology, thermal characteristics, and rheological properties of the PLA nanocomposite were examined. The PLA microcellular foaming nanocomposite was prepared using supercritical CO2 foaming. Furthermore, the foaming conditions were optimized, and the effect of NCC surface modification, as well as the content and interfacial force of nanoparticles, on the foaming efficiency and cell pattern of composites was examined. The results suggested that a sufficient number of nanoparticles increased the nucleation site for foaming and played a vital role in heterogeneous nucleation in the foaming system. An ideal microcellular foaming material with an average cell diameter of 6.97 μm and a cell density of 6.05 × 109 cells/cm3 was prepared when the foaming temperature was 110°C, the foaming pressure was 18 MPa, the incubation period was 2 h, and the mNCC content was 6%.