The invention provides a constant humidity machine. The constant humidity machine comprises a runner capable of being driven by a first drive device to rotate, a heating device arranged near the runner so as to heat water on the runner and generate water vapor, a condenser arranged near the runner so as to receive the water vapor from the runner and cool the water vapor into condensate water, a water tank arranged near the condenser so as to receive condensate water from the condenser, a water spraying device arranged near the runner and connected with the water tank so as to spray water in the water tank to the runner, and a circulating fan used for providing circulating air for the constant humidity machine. The provided constant humidity machine can achieve dehumidification and humidification functions at the same time, the structure is simple, and an even humidification effect can be achieved in the humidification mode.