A mismatch compensating device includes: a signal generator, synchronously outputting first and second signal; a gain and phase compensator, processing the first and second signals according to a gain parameter and a phase parameter to generate compensated first and second signals; a DAC, performing a digital-to-analog conversion on the compensated first and second signals to generate first and second analog signals; an analog front-end circuit, processing the first and second analog signals to output a joint signal; a mismatch detecting circuit, detecting the power of the joint signal to generate a detection result; a frequency-dependent mismatch compensator, compensating at least one of the first and second signals; and a control circuit, setting the gain and phase parameters and a parameter of the frequency-dependent mismatch compensator according to the detection result to compensate frequency-independent gain and phase mismatch and a frequency-dependent mismatch response.