A mixed-signal control apparatus of a modulation system is provided. The mixed-signal control apparatus includes a digital-to-analog converter (DAC) unit, an analog-to-digital converter (ADC) unit, and a processing unit. When a quantization step of the ADC unit is qADC, a quantization step of the DAC unit is qDAC, and a transfer function of the modulation system is G0, qADC≦G0qDAC. A controlled variable is finely converted by using the ADC unit having a very high resolution. In such a way, it can be ensured that data in an analog domain can be transmitted to a discrete domain with the least distortion, thus achieving a robust and simple control.