Abstract: Errors in instrumental voltage and current transformers can lead to amplitude and power factor angle deviation measured in synchronous phasor measurement unit (PMU), and result in abnormality in active and reactive power measurement. However, detection methods for abnormality in reactive power measurement are rarely studied. In this paper, the fact that the power factor angle deviation would induce abnormal measured reactive power loss is derived. Combined with the estimation of transmission line (TL) reactive power loss independent of phase angle, a detection method for reactive power loss measurement abnormality is proposed. In detail, first, the inconsistency of reactive power measurement of a double circuit line is introduced. Then, the fact that power factor angle deviation would influence the reactive power measurement more significantly than that of active power, and induce abnormal reactive power measurement, is theoretical explained with sensitivity. Furthermore, an estimation for the TL reactive power loss based on PMU amplitude measurement and TL parameters (independent of phase angle) is derived and quantitatively analyzed. Moreover, a detection method for the reactive power loss abnormality of TL is proposed, which can also be applied in detection for reactive power measurement abnormality of single bus of a TL. The proposed method does not require pre-calibrated or high-precision transformer as reference, and can realize online detection. Finally, the results with simulated and measured measurements data show the effectiveness and practicality of the proposed method, respectively.