Abstract: In response to the abnormal state of the nuclear instrumentation cables outside the nuclear power plant during the cold shutdown phase, insulation performance inspection and aging status evaluation on nuclear instrumentation cables were conducted. Linear resonance analysis, time-domain reflection(TDR) test, and dielectric loss spectrum test were used to locate local defects and evaluate aging status of nuclear instrumentation cables. The results demonstrated that the multi-dimensional diagnostic methods were simple and convenient, with defect localization being accurate and efficient. Finally, the condition evaluation results of the nuclear instrumentation cables and the improvement suggestions for the problems existing in cables service process were given.