The polyvinyl alcohol (PVA) polarizing films were prepared by the flow-casting method. The crosslinking or uncrosslinking iodine-PVA polarizing films with different polymerization degree were obtained after the determination of iodization and crosslinking time by orthogonal experiments. The transmittance was measured and the polarization degree of PVA polarizing film was calculated by using UV-Vis before and after hygrothermal aging tests. The chemical morphology and crystal structure of the polarizing films with different polymerization degree were characterized by using Raman and XRD. The molecular structure of the polarizing films with different polymerization degree before and after hygrothermal aging was analyzed by using FT-IR, and the effects of polymerization degree and crosslinking on the optical properties and aging resistance of the polarizing films were systematically investigated. The results show that as the polymerization degree increases, the adsorption of PVA films to iodide ions improves, the proportion of I₅^- in films increases, and the hygrothermal resistance aging performance of polarizing films becomes better as well. From the perspective of the change of polarization degree of polarizing films, the higher degree of polymerization is, the better optical performance and the lower failure rate of aging are. Under the same polymerization degree, the hydrolysis resistance and the hygrothermal resistance aging performance of the polarizing films are significantly improved after crosslinking. High humidity promotes the disorientation of polarizing films and the hydrolysis of borate ester bond in crosslinking structure, resulting in the destruction of films structure and iodide ions orientation. The iodide ions loss is the main reason for the failure of polarizing films.