With the development of infrared technology, characterizing the response nonlinearity of infrared detectors has become a key focus for domestic and international research. However, traditional test methods cannot effectively distinguish between linear and nonlinear responses, making the accuracy of nonlinear testing limited. By using harmonic signals to characterize the impedance nonlinearity of photoconductive detectors and extract their nonlinear components, the interference of linear signals can be excluded and the accuracy of the tests can be increased. In this study, two detectors of different materials and structures were tested using the harmonic method. The results show that the harmonic signals due to the impedance nonlinearity of the detector prepared from the liquid-phase epitaxial material are approximately 100 times higher than those of the bulk-material detector. This may be due to the inconsistent effect of thermal excitation on different detectors, resulting in different degrees of nonlinearity. The feasibility of the harmonic method in impedance nonlinearity testing of photoconductive detectors is demonstrated by a combination of experimental and mathematical analyses.