W ith regard to the flaw of the preamplification circuit for common infrared detectors ，it proposes the design of a double output differential amplifier circuit after analyzing the output signals of the pyroelectric infra‘red sensor，based on the A／D conversion technology．The amplifier circuit is supplied single power，resorting to the signal processing by MCU．The experimental results show that the amplifier circuit can efectively reduce the influenceof common-mode noise inside the detector and the WaVC motion of environmental temperature on output signals，and that the equency and amplitude of output signals reveal information on the velocity and direction of the human body’s movement．