The present invention relates to an eco-friendly lightweight substrate material for the automotive interior, characterized in that isocyanate or epoxy is added to enhance the function of a substrate material having a sandwich-type structure for the automotive interior including natural fiber that is vulnerable to high temperature and humidity conditions, preventing degradation of physical properties by water-impregnation into the natural fiber and thus enhancing the humidity-resistance and strength of a natural fiber reinforcing layer; and the substrate material is continuously prepared in a thermoplastic foam sheet core layer by thermal-laminating. The substrate material prepared according to the present invention is an eco-friendly material, also is capable of weight lightening by weight reduction, and is excellent in humidity-resistance and strength, thus providing for application to various industries such as train interior, aircraft interior, and architectural interior as well as automotive interior.