Time-frequency domain reflection (TFDR) method has the advantages of maturity,convenience and high resolution,so it is widely applied to locate defects in various cables. To ensure the fracture defect of the instrumentation and control (I&C) cable can be effectively discriminated and precisely located in nuclear power plant (NPP),an improved TFDR method is proposed by intervening thermal stress in this paper . Subsequently,different degrees of fracture defects are simulated in a 50-meter multi-core I&C cable,and traditional TFDR and improved TFDR methods are used for detection and verification. Comparing the detection results of both,the peak localization of improved TFDR method for weak defect fracture is improved to 0.2,which is 2.5 times of the traditional TFDR method,and the localization error rate is less than 2.5%. Compared to the traditional TFDR method,improved TFDR method utilizes the thermal stress fluctuation variance curve as the basis for defect identification,the identification power of weak fracture defects can be substantially improved. The original localization distance can be corrected to improves the accuracy of weak fracture damage in improved TFDR method. So the effective identification and localization of weak defects can be further achieved in improved TFDR method.