To investigate the influence of the degree of crosslinking of XLPE material on its electrical properties, XLPE samples with different degrees of crosslinking are produced and breakdown experiment are applied on the samples, and the changes of the breakdown strength of the samples in different regions of degrees of crosslinking are analyzed. First six groups of XLPE samples with different degree of crosslinking were produced by T-t (temperature-time) controlling method, and the crystallinity of the samples are measured by the Differential Scanning Calorimetry (DSC) instrument. The breakdown strength of the samples were measured by an alternating current transformer. The experimental results show that with the increase in the degree of crosslinking of the samples, the breakdown strength of the samples first increase and then decrease. When the degree of crosslinking of the samples were 0%-75%, the breakdown strength of the samples increase by 2%. When the degree of crosslinking of the samples were 75%-88%, the breakdown strength of the samples increase by 21.2%. When the degree of crosslinking of the samples were 88%-92%, the breakdown strength of the samples decreases by 5.3%. It is believed that the breakdown strength of the XLPE material is related with the degree of crosslinking and the uniformity of the material. When the degree of crosslinking of the samples is low, the breakdown strength of the samples is mainly determined by the degree of crosslinking. With the increase in the degree of crosslinking, the breakdown strength of the samples can be promoted. However, since the partial parts of the material can be nonuniform, the increment of the breakdown strength of the samples increase little. When the degree of crosslinking increases to 75%, dense, uniform three-dimensional network structure can form in the material, which leads to the significant increase of the breakdown strength. When the degree of crosslinking further increases, the decrease amplitude of the crystallinity can be large, and the structure of the partial material become nonuniform, which result in the decrease of the breakdown strength.