The accuracy of overvoltage measurement system is a perquisite to evaluate the insulating performance of the line.This work mainly focuses on the influences of the input impedance of oscilloscope.Firstly,the principles of switching overvoltage are analyzed,followed by a brief introduction to the structure of the measurement system.To facilitate the analysis,the transfer function is employed to study the influence of the input impedance.Using the Laplace Transform,the transfer function of the measurement system is obtained,and two typical cases are considered,including the response to a unit step function and a sine function.The effect of the input impedance on the measurement system is discussed.Finally,to further confirm the theoretical analysis,an experimental study has been carried out at a substation.Two channels of a waveform recorder are selected to make a comparison,and a series of experiments are performed.Both the theoretical analysis and the experimental results demonstrate that the input impedance influences the accuracy of the measurement system significantly.A system with higher impedances will offer higher accuracy.Moreover,the input impedance can be used as an indicator in the calibration of overvoltage measurement system.