Sampling delay is an important index of digital sampling. Proceeding from the composition of each link of digital sampling

the characteristics of sampling delay and the influence on the secondary system are analyzed in detail. In view of the problems with the widely adopted steady power frequency testing method

frequency changing principle-based method of delay testing is put forward. This method can accurately measure the non-integral period and integral period delay of digital sampling. The correctness of the frequency changing method is verified by simulation and actual testing. Finally the specific implementation of delay testing by the frequency changing method is proposed for project application.