An instrument is proposed for nitrate in situ monitoring in deep sea. It consists of a UV spectrophotometer, a deuterium lamp of high stability and low power consumption, a rugged and high-throughput retro-reflection probe and a center control board with large data storage capacity. With a measurement precision of nitrate concentration of 1μmol/l and a resolution of 0.5μmol/l, the instrument is able to measure nitrate concentrations from 0.5μmol/l to 320μmol/l. The system can automatically deduct the dark current to rectify the effect of temperature drift and correct the overall absorbance drift caused by instrument drift through integration of relative absorbance (IRA) algorithm. The system can endure the pressure at up to 1500m deep sea and carry out automatic long-term in situ detection of nitrate with high stability without needs for any manual intervention.