A detecting circuit of an infrared thermopile is introduced. Then the output signal and noise of the detector are analyzed. With calculating the inherent noise of the analog signal chain, the amplifying and conditioning method of weak signal are studied and a new method of noise match is proposed. The circuit design of each step is also given out, which has been simulated with Pspice. The test result shows that the design has achieved a good result in noise suppression and offset compensation, which can detect minimal 3.5 μV weak DC signal. This design can be widely used in infrared spectrum detection and weak DC signal detection of μV class.