The invention discloses a nondestructive detection method of a composite insulator. The nondestructive detection method of the composite insulator comprises the following steps: coating the surface of a to-be-detected composite insulator with an ultrasonic coupling agent; outputting a pulse signal by an ultrasonic testing system; uploading the pulse signal to an ultrasonic transducer by the ultrasonic testing system and converting the pulse signal into an ultrasonic signal; transmitting the ultrasonic signal by the ultrasonic transducer to the to-be-detected composite insulator to generate an ultrasonic echo signal; receiving the ultrasonic echo signal by the ultrasonic testing system, converting the ultrasonic echo signal into an electric signal and determining resonant frequency of the electric signal; and analyzing the viscidity condition of the interface of the to-be-detected composite insulator by the ultrasonic testing system according to a resonant frequency and preset resonant frequency comparison diagram. According to the embodiment of the invention, the composite insulator not containing macroscopic air gap can be subjected to nondestructive detection.