In this paper, the measurement error of capacitor voltage transformer (CVT) under harmonic condition is analyzed and researched through theory and test. According to the structure and working principle of CVT, the frequency response characteristic of the ratio and theoretical expressions of the measuring error of harmonic amplitude are deduced. On this basis, the influence laws of different factors on the measurement error are analyzed. Finally, the experimental platform of CVT measurement error is designed, and the correctness of the analysis conclusions are verified through experiment.