The crude tannin extract of Coriaria nepalensis leaves was prepared by using C. nepalensis leaves as raw materials, and then the crude tannin extract was purified by resin purification method. Based on static and dynamic adsorption and desorption experiments, the optimum purification process conditions of crude tannin extract were established. Further, the purified tannin was used as a cross-linking agent to crosslink with the gelatin material to prepare a tannin gelatin composite film material. The mechanical properties, light transmittance and antioxidant properties of tannin cross-linked gelatin films were investigated by FTIR, UV-Vis, electronic universal testing machine, hydroxyl radical scavenging method and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging method. The results showed that the optimum process for preparing the purified tannin from C. nepalensis leaves was as follows: AB-8 macroporous resin was used as the adsorption material, the sample concentration was 2.0 mg/mL, the flow rate was 3 BV/h, and the pH was 5, the tannin solution was 4 BV, and after 3 h of adsorption, 5 BV of 60% ethanol solution was added at a flow rate of 3 BV/h for elution. The purity of tannin from C. nepalensis leaves could be increased from 27.3% to 65.6%. Compared with the blank gelatin film, the elongation at break of the tannin cross-linked gelatin film prepared by adding 2% purified tannin of gelatin mass increased by 81.6%, and the tensile strength increased by 94.9%. The tannic cross-linked gelatin film had strong absorption of ultraviolet light (T <10%) in the range of 250~390 nm, and the transparency value increased from 0.72 to 0.98. Compared with the blank gelatin film, the DPPH free radical scavenging rate of tannin cross-linked gelatin film increased from 5.4% to 42.3%, and the hydroxyl radical scavenging rate increased from 3.3% to 37.6%.