The phenomenon of response nonlinearity of infrared detectors and its characterization has gradually become a new hotspot for domestic and international research with the development of infrared technology, but the testing methods of nonlinearity have been limited by the accuracy. Using harmonic signals to characterize the impedance nonlinearity of photoconductive detectors, tg by extracting the nonlinear components, the interference of linear signals can be excluded and the accuracy of the test can be increased. Two detectors of different materials and structures were tested using the harmonic method, and it was found that the harmonic signals due to impedance nonlinearity of the detector prepared from the liquid-phase epitaxial material tested were about 100 times higher than those of the bulk-material detector, which 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 analysis.