Photo acoustic spectroscopy technology is increasingly used for its high sensitivity characteristics in the detection of SF6 decomposition in substations and medical diagnosis. A kind of low noise design of weak signal detecting circuit is designed.According to the model of noise analysis,the system total noise and SNR are calculated based on the theory of low noise design.Then the pre amplify circuit and signal conditioning circuit are designed based on the analysis of the operation amplifiers.The experimental results show the detection circuit can translate the photo acoustic signal to the electric signal steadily,which has the high SNR, high reliability and high stability.