The accuracy of switching overvoltage measuring system is significantly affected by the input impedance of oscilloscope. First, the principle of switching overvoltage is presented, followed by a brief introduction to the structure of measuring system. Using the laplace transform, the transfer function of the measuring system is then derived. Its frequency response, ranging from DC to 1 GHz, is investigated in detail. The influences of the input impedance on the amplitude and phase angle responses are clearly revealed. Two typical cases, i.e. unit step response and sinusoidal step response, are also considered in the study. To validate the above analysis, a series of experiments has been performed at a substation. Both the theoretical analysis and the experimental results indicate that the input impedance affects the accuracy of switching overvoltage measuring system significantly. In the case that the impedance is rather small, the voltage waveform displayed on oscilloscope would be considerably distorted from the one stressed on the line. The impedance should be greater than 10⁷ Ω to satisfy the accuracy requirement. On the other hand, it can be also used as a direct indicator to assess the performance of oscilloscope.