The accurate measurement of over － voltage is very important to ensure the safety and stability of power grid． Therefore，the frequency response characteristics of over － voltage monitoring device employing capacitor voltage transformer ( CVT) under different load conditions are tested and analyzed，and the effect of load change on frequency response characteristics without and with interior low － voltage capacitor is studied． The results show that: for the case without interior low － voltage capacitor，when the secondary side of CVT at no － load or rated load，the frequency characteristics are roughly the same，but when the load is heavy，the frequency characteristics are greatly different from the situation under rated load． When there is an interior low － voltage capacitor，the presence of interior low － voltage capacitor has almost no effect on the frequency characteristics of CVT under different load conditions，and there is also little effect on the frequency characteristics in high － voltage side of interior low － voltage capacitor under rated load and no － load conditions．