The measurement accuracy of capacitor voltage transformer (CVT) affect the accuracy of electric energy measuring and reliability of relay protection action. In order to study the accuracy of CVT measurements under the harmonic conditions, and to improve the measurement accuracy of the CVT to provide reference,this paper has obtained some conclusions based on the equivalent circuit model of CVT. The expression of CVT measurement ratio under the harmonic conditions is derived, and the effect of excitation impedance and capacitance divider ratio of the measurement accuracy under harmonic conditions has been researched and been verified by simulation analysis.