Electrode stability is an important index of geo-electrical observation,and usually is evaluated by measuring its earth resistance. In this paper,we analyzed the existing measurement methods,and believed that the method of electrical measurement is more reasonable. An electrical measurement method considering cable resistance then is proposed. Taking Jiangning Station as an example,several methods were used to measure the earth resistance,and compared the result errors from different methods. We found that the electrical measurement method is more suitable for deep borehole observation. Furthermore,we discussed the grounding resistance out of standard and the earth resistance of electrodes at the same depth in different boreholes as well as at different depths in the same borehole,in which the big difference may be related to the formation humidity around the electrode and the porosity of the medium. The disadvantages of electrical measurement method are also analyzed in the paper. Finally,we suggested that when the earth resistance exceeds the standard,the stability of the electrode should be judged by its variation range.