Direct expansion (DX) air conditioning (A/C) systems are most commonly used in residential buildings in hot and humid subtropics. They are normally equipped with single-speed compressors and supply fans, relying on on-off cycling compressors to maintain indoor dry-bulb temperature only, leaving indoor humidity uncontrolled. The reason for this situation is the mismatching between an equipment sensible heat ratio (SHR) and an application SHR. This paper reports a study on this mismatching problem for DX A/C systems used in subtropical residences. With the aid of a building energy simulation program, EnergyPlus, the indoor humidity control problem with two SHRs mismatching is discussed. An experimental station has been built, humidity control strategy with variable-speed compressor and supply fan is presented; and a complete simulation model representing DX systems, based on the design parameters and test data of the experimental station is developed.