To meet the requirement of real-time spectral measurements and feature extraction of sky background radiance, a measuring instrument for spectral radiance of sky background was developed based on fiber optical spectrometer, to acquire spectrum from the wavelength of 340 nm to 1030 nm, which could both operate at fixed direction mode and whole sky scanning mode. Besides the overall structure, the details of optical component, rotating system, sun tracing module, temperature maintain system, software design, working process and calibration method were also presented. The calibrated test results were analyzed to prove the instruments’ reliability, and shows that it provided a way for real-time spectral measurements of sky background radiance.