The invention discloses a nondestructive measuring method for online measurement of pitch changes in a superconducting cable during back-twist on the basis of infrared thermal imaging. Under the sameillumination condition, the pitch changes of the cable are measured by obtaining infrared pitch images between sub-cables, and then the back-twist angle of the cable is obtained through formula conversion. Under the condition that a conductor is not damaged, the pitch length and changes of the sub-cables are determined by measuring the sizes of infrared gap images between the sub-cables on a measuring negative film, and finally, the accurate value of the back-twist angle is calculated. The method has the advantages of being small in error and high in efficiency.