Abstract: A polygraph biofeedback instrument that can acquire four channel psychophysiological signals is developed based on the technology of biofeedback. The acquired signals (electromyogram, galvanic skin response, respiration, pulse) are transmitted into the computer after magnification and filtration. The computer then processes the data. As a kind of feedback information, the signal charts are displayed on the screen. The basic construction of the polygraph biofeedback instrument and its features are introduced. An analysis of the electromyogram features and its methods of extraction are also given. Its results in actual application show that the polygraph is practical and reliable.