The relationship between the permittivity distribution and the measured capacitance in electrical capacitance tomography (ECT) is described by the sensitivity distribution which directly affects the image reconstruction. The sensitivity distribution is mainly computed according to its definition which is complicated and time-consuming. We analyze relationship between the sensitivity distribution and the electric potential distribution in ECT. The calculation formulae are pressented. The method is simple and fast. Simulation results show that the sensitivity distribution obtained by this method is consistent with that of the traditional method.