The active transport system is considered as a Maxwell demon, this demon uses short range force (affinity) to recognize specific ion so as to move the ion selectively to the other side of the membrane. A detailed calculation is given to prove that this demon is not in contradiction to the second law of thermodynamics. When the conformation of the carrier is changed from low affinity (K2) to high affinity (K1), there is an energy supply. The limitation of the uphill transportation is determined by K1/K2=[I]2/[I]1, where [I]1 and [I]2 are the concentrations of I at' side 1 and side 2 of the membrane respectively. The general rule that information=neg-entropy is hold for this demon.