During the manufacturing process of HVDC cables, the insulation layer's surface often exhibits micro-sized defects. These defects can affect the electrical properties of the power cable, thereby influencing the aging process, insulation breakdown, and reliability of the HVDC cable in high-voltage direct current transmission systems. To investigate the influence of different surface conditions on the electrical characteristics of HVDC cables, this paper conducts 2D simulations on XLPE insulation material samples of a mini HVDC cable. The simulations incorporate various surface conditions of the insulation layer, including an ideal smooth surface, independent defects, and clustered defects, to examine the variations in internal electrical characteristics such as space charge distribution and electric field distribution inside the insulation layer. The simulation results indicate that the presence of non-ideal surface conditions significantly affects the electrical properties of the insulation layer, as well as adversely impacts the reliability of the HVDC cable in power transmission systems. The differing degrees of influence from various surface conditions and their causes are also discussed in this paper.