Abstract: The conservative design of insulation thickness increases the production cost of the cable, meanwhile, the increase of the thickness leads to the decrease of breakdown strength and the degradation of the cable performance. Therefore, the study of the effect of thickness on breakdown strength is of great significance. This paper selected domestic cross-linked polyethylene（XLPE） which was used in high voltage direct current cable insulation as research object, performing the DC breakdown test and space charge test at 30, 50 and 70℃ to investigate the influence of temperature on the thickness effect of the breakdown strength. The result shows that the DC breakdown strength of XLPE decreases with the increase of temperature. The relationship between the breakdown strength and thickness of XLPE is in accordance with the inverse power law at different temperatures, and the exponential factor changes with temperature, which is 0.41 at 30℃, 0.71 at 50℃, and 0.56 at 70℃. The space charge density and electric field distortion of XLPE have a positive correlation with the thickness at 30℃, which leads to the decrease of breakdown strength. The space charge accumulation and the electric field distortion increase with the increase of temperature, and this change is more pronounced in thin samples. These results indicate that the thickness effect of the breakdown strength of XLPE at different temperatures is closely related to the change of free volume of polymer under heating, the accumulation of internal heat and the secondary effect of space charge.