Abstract: For substation protection, the correctness of the indirect lines between current transformers and protection devices is crucial. This article mainly discusses the relevant issues of on-load phasor measurement, focusing on three important aspects in the process of protection phasor measurement: the correctness of TA transformation ratio, the correctness of current phase sequence, and the correctness of TA polarity. At the same time, the relevant factors that may cause incorrect phase measurement of protective protection during the actual on-site measurement process were also discussed.