The performance change and action mechanism of cross-linkable polyethylene material for cable insulation during storage are still unclear. This article is based on two storage methods commonly used in laboratories — tin sealant and plastic sealant. By characterising the dielectric properties and crystalline structure of XLPE samples prepared from the new cross-linkable polyethylene granules and the granules of same brand with two years’ storage by the above two storage methods, the effect and mechanism of storage on the properties of cross-linkable polyethylene material are studied. The experimental results show that the change in properties of the cross-linkable polyethylene material during storage is due to the decomposition of the cross-linking agent contained in granules. The cross-linking degree of the XLPE samples obtained from the granules after shelving for two years decreased significantly, the thermal extension properties deteriorated, the crystalline properties and crystalline structure are slightly improved, and the dielectric properties are improved. With the effect of light, the decomposition of the cross-linking agent is more serious, and the performance changes are the most obvious.