In order to improve the water stability and mechanical properties of polyvinyl alcohol (PVA ) films, PVA-(PAA-CNC) composite films were prepared with polyvinyl alcohol and cellulose nanocrystals (CNC) which were modified with poly(acrylic acid) . The structure of the composite films was characterized by Fourier transform infrared spectroscopy (FT-IR)and scanning electron microscopy(SEM). The physical characteristics of the films, including the water stability, mechanical and transparency properties, were investigated by contact angle test, tensile test and transmittance test. The results show that compared with pure PVA films, PVA-(PAA-CNC) composite films maintain a high transmittance in the visible light band with only 5.6% decreased and the surface hydrophobicity of the films are improved with the contact angle increased by 40°. When PVA is modified with CNC alone, the agglomeration of CNC in the PVA-CNC films is obvious, and the mechanical properties of the films are sensitive to environmental humidity. When PVA is modified with CNC and PAA, the agglomeration is significantly inhibited, meanwhile it can maintain good mechanical properties under 100RH% wet condition with its elongation at break greatly improved.