Abstract: Humidity is one of the most important interference factors during the measurement of isothermal relaxation current for the field distribution cable, and it may significantly affects the evaluation of insulation state based on relaxation current. In this paper, the retired three-core outdoor terminal was taken as a sample. The isothermal relaxation current of sample was detected at different humidity environments and the error analysis state evaluation was carried out. The results show that, after the detection system DC power supply outputs 1 kV, the isothermal relaxation currents of A, B and C phase cables and the actual voltage V_c of the cable cores present an overall rapid decrease trend with the increase of ambient humidity, which indicates that the decrease of the isothermal relaxation current was mainly related to the decrease of the creeping resistance R_h of insulator umbrella skirt. Isothermal relaxation current in A, B and C phase cables can all be fitted by the third-order exponential model. The calculated aging factor A are all larger than the "deterioration" grade line 2.10 in the German criterion. In addition, there are obvious differences in aging factor A values of each phase, which indicates that the aging degree of the cable insulation of each phase is different. This phenomenon may be related to the electrothermal aging process of the insulation layer caused by the different load of each phase line. When the ambient humidity is below 49%, the error of the aging factor A calculated by the isothermal relaxation current method with different humidity is within 6%. However, when the humidity is further increased, the insufficient polarization electric field will lead to an excessively high error of the aging factor A and low accuracy. Therefore, attention should be paid to the influence of environmental humidity in practical applications. It is recommended that the relative humidity of the environment around the cable joint be controlled below 49%.