A multi-spectral infrared radiometer system is designed that can be used to measure sea surface temperature automatically, whose resolution ratio and accuracy are better than 0.1 ℃ and ±0.5 ℃, respectively. The system adopts four spectral bands, i.e., 3.5～4.0 μm, 8～13 μm, 10.3～11.3 μm, and 11.5～12.3 μm. Its inner optical system carries out real time calibration by two blackbody reference cavities with different temperatures, which promises that the high accuracy of an infrared radiometer is maintained even when optical components are moderately contaminated by salt deposition. Moreover, a protection shutter which could be automatically triggered by a sensitive rain gauge is designed to resist against foul weather. Finally, the accuracy and stability of the system are tested by laboratory calibration and shoreside experiment.