The invention relates to the technical field of rail traffic, and discloses a light environment-friendly high-speed train side wall plate and a preparation method thereof. The light environment-friendly high-speed train side wall plate comprises an outer side wall plate, an inner side wall plate and a windowsill insert; the outer side wall plate and the inner side wall plate are connected togetherin a stacked mode, and a window is formed in the middle; the inner side wall plate comprises an inner side wall plate body with an upper opening, and a bridging plate connected to the upper opening of the inner side wall plate body; the windowsill insert comprises a windowsill plate connected to the lower edge of the window; the outer side wall plate, the inner side wall plate body, the bridgingplate and the windowsill plate are each made of a die pressed bast fiber composite plate, wherein the die pressed bast fiber composite plate comprises a front face layer, a middle layer and a back face surface which are sequentially connected in a stacked mode; and the middle layers are bast fiber felt layers made of mixed fibers of bast fibers and polypropylene fibers, and the front face layers and the back face layers are both nonwoven fabric layers. The light weight of the high-speed train side wall plate is achieved, and environmental protection performance of the side wall plate is further improved.