Abstract: Measurement accuracy of instrument voltage transformers directly affects the safety, stability and economic operation of the power grid. In fact, the existing evaluation methods based on large-dimensional random matrix cannot reliably evaluate transformers due to the influence of primary voltage fluctuations, this paper proposes a method for evaluating the error condition of voltage transformers based on differential processing using adaptive differential interval (DP-ADI) and measurement consistency of same phase (MCSP). According to the time series theory, the primary fluctuation is divided into non-stationary fluctuations caused by definite factors and stable small stationary fluctuations caused by random factors. Moreover, a stabilized method based on adaptive differential interval is put forward to weaken the influence of non-stationary fluctuations. In addition, a difference evaluating indicator based on the measurement consistency of same phase is established to diminish the influence of stationary fluctuations. The simulation experiments and field application results suggest that 0.1% error of voltage transformers can be estimated by using the improved evaluation method based on large-dimensional random matrices.