In near infrared spectroscopy for detection of industry systems, the useful information weak signal is often submerged in strong background noise. The lockin amplifier could be used by weak signal detection technology to achieve the extraction of useful signa1. Weak signal detection theory was described, and the outputs of the different reference signals were compared. A lockin amplifier was designed using the AD630 as associated multiplier to extract the useful information in strong background noise. The test results indicated that the circuit structure was simple with high linearity of outputs, and the SNRI value was 62 dB.