The invention discloses a composite base material root resistant waterproof sheet material. The sheet material is a multi-layer coextrusion composite sheet, including two outer layers and at least a three layer structure between the two and formed by alternately stacking of a buffer layer and a matrix layer; the outer layer is non-woven fabric; the buffer layer is modified polymer mixed polyethylene; the modified polymer mixed polyethylene comprises 40%-60% of linear low density polyethylene, 25%-35% of metallocene and 15%-25% of ethylene acrylic acid; the base material is a at least seven layer structure formed by alternately stacking of a nylon layer and a heat sealing layer, wherein the nylon accounts for more than 30% of the total weight of the matrix layer. The invention also discloses a preparation method of the sheet material, and the method employs fusion coextrusion. The sheet material of the invention has strong root resistant performance and waterproof performance, and is light and transparent, and suitable for application.