Abstract: The reliability of the cable plays an important role in the stable operation of the power system. However, the dielectric strength of cables declines under the effect of AC electrical and thermal stresses when in operation. The gradual degradation processes caused by composite stress on dielectrics, known as electrical aging, have a close relationship with the reliability of electrical equipment, though are not completely recognized. During operation, alternating voltage will lead to the change of internal charge distribution of cable insulation, resulting in local electric field distortion and aging of insulation materials. At present, the most widely used cable insulation material is XLPE, so it is necessary to study the space charge distribution of XLPE high AC voltage. 1. Experimental scheme The research object of this paper is XLPE cable section with 230 μm thickness. The experimental scheme is to apply AC electric field, 35kV/mm (RMS), for 6h on the XLPE cable peeling sample, under the temperature of 103 °C. Space charge waveforms were produced by averaging more than 500 sinusoidal periods in one time, and the averaged space charge profiles in 32 phase angles were achieved with an interval of 11.25°. The space charge measurement system is shown in Figure 1. 2. Experimental result The space charge density at the angles of 90° and 270° has the largest magnitude, but their charge density varies in very small scale in the 6h application time of stress. Therefore, a new data analysis method, all-phase averaged method, was used for data processing. The method averages the space charge density of the 32 phases at the same time to obtain the change of the total space charge density in a period. The waveform obtained by the method can be observed that the space charge density increased with time. After the AC voltage-off, the space charge density decreased in 180s. 3. Conclusion The results show The essence of the all-phase averaged method is to clearly reveal the stationary charge, which grows gradually with the increase of application time under ac stress. The stationary charge density has a close relationship with the application of ac aging stresses and requires particular attention. This work discovers the characteristics of space charge under ac stress, and contributes to the understanding of space charge features of cable insulation in real operation condition.