The volt-ampere (V-I) characteristic curve obtained using single probe diagnostic system for collisionless plasma will be distorted when the system is applied to collisional plasma. The V-I characteristic curves of collisionless and collision inductively coupled plasma is obtained at the gas pressure of 20 Pascal and 400 Pascal, respectively. The distortion phenomenon was studied by measuring the V-I characteristic curves at different positions of the probe and introducing interference electrode to adjust the level of overall electrical-neutrality constraint. The inferential and experimental results show that the overall electrical neutrality in collisional plasma will compose constraint on the single probe diagnostic system for collisionless plasma. Moreover, it is concluded from experiments that the plasma will adjust the potential of itself, which makes the potential of vacuum chamber wall as reference, in order to satisfy the overall electrical neutrality, and that the V-I characteristic curve obtained by single probe is not the V-I characteristic of probe sheath predicted by theory.