High-sensitive detector arrays in terahertz(THz) band have great application value in many fields. Quantum capacitance detector(QCD) is one kind of direct detectors with single-photon detection and array capabilities in THz band, which uses frequency division multiplexing, and the array readout technology is relatively simple. For high-sensitive detector arrays, reliable readout technology is the basic guarantee for its superior performance. In this study, QCD signal is measured and characterized by homodyne readout technique. As microwave in-phase quadrature-phase(IQ) mixer is an important component of the homodyne readout circuit, the IQ mixer is characterized and calibrated, and its measurement reliability is improved by eliminating the impact of its imbalance on signal readout. Under this calibration method, the quantum capacitance response signal of QCD is measured, and the results show that the QCD response signal is periodic, which is highly consistent with the expected quantum capacitance effect. In addition, the constructed readout circuit can also be used for signal readout of extremely low temperature(below 15 mK) detectors such as microwave kinetic inductance detector(MKID) arrays, which lays a good foundation for the development of high-sensitive THz detector arrays and has good application value.