A method for detecting a tension force defect of a steel cable includes the following steps: step S10: providing an excitation sensor on a first position of a steel cable to be detected and providing a detection sensor on a second position of the steel cable; step S20: obtaining a value of a tension force on the steel cable and obtaining a first display diagram corresponding to the value of the tension force; step S30: loading a white noise signal on the excitation sensor through a power amplifier; step S40: acquiring a detection signal collected by the detection sensor; step S50: uploading the detection signal to a personal computer (PC) and performing a Fast Fourier Transform (FFT) process on the detection signal; step S60: determining whether there is a defect in the steel cable, and if so, performing step S70; step S70: determining a defect position of the steel cable.