CO2 laser heterodyne detection is one of the promising methods for civilian and military radar. In this paper, the nonlinear four-frequency heterodyne detection have been analysed theoretically. In experiment, the nonlinear fourfrequency heterodyne detection was realized and the results obtained coincides with expected one. Square amplifier is the key element for the processing of electric signals from heterodyne detector in four-frequency system. The main principle is described and a practical device had been made used in the experiment.