Indoor air humidity is an extremely important environmental parameter that can significantly affect the energy consumption of a building, its structure, the chemical emissions from building materials, as well as occupants’ comfort and health. How to control indoor relative humidity has been the subject of much attention in the past decade. This paper expounds the passive and active control strategies, introduces a new control theory with a corresponding control scheme, and gives a simple simulation of a typical controlled room. The results of the simulation showed that a new control scheme gave better transient and static characteristics than a conventional PID control scheme.