The invention belongs to a ventilation humidity regulating unit, and particularly relates to a series single cold and heat source ventilation humidity regulating unit. The ventilation humidity regulating unit comprises an indoor unit and an outdoor unit, wherein the indoor unit comprises an air inlet, a filter screen assembly, an evaporator, a reheat condenser, a humidifier, a fan and an air outlet which are arranged in sequence; and the outdoor unit comprises a four-way valve, a compressor, a condenser and an outdoor expansion valve, wherein the compressor, the condenser and the outdoor expansion valve form a loop with the evaporator, the evaporator, the compressor and the condenser are connected with the four-way valve, a radiator is arranged on the condenser, the reheat condenser is arranged between the condenser and the evaporator in series through a pipeline, and a reheat expansion valve is arranged on the pipeline where the reheat condenser is located in parallel. According to the series single cold and heat source ventilation humidity regulating unit, a single cold and heat source is adopted to process air, the heating temperature of the reheat condenser is controlled by regulating the flow of the reheat expansion valve, stepless regulation can be realized, the regulation process is simple and flexible, and the indoor unit and the outdoor unit are connected through two pipelines, so that the installation and connection difficulty is reduced.