The surface-action is Considered here and the transfer characteristic of Faraday cup is derived theoretically. The analysis shows that there is an eigen frequency ω_c，which is related to the structure of the cup. When ω(measured signal's frequency) 《ω_c，the transfer characteristic function is a constant; but when ω≥ω_c，the function is the sum of a constant and a decreasing oscillation term. On the latter condition，the cup would cause the distortion of the measured signal.