A calibrating method for the irradiance responsivity of 200~300 nm spectral instruments using 150 W deuterium was introduced in detail. This method overcame the troubles caused by the weak ultraviolet response of most spectral radiometer and poor signal-to-noise ratio. A spectral irradiance calibration unit based on parallel light was established to meet the calibration conditions of some remote sensing spectral radiometers. The calibration unit got the standard value of spectral irradiance by irradiance transfer

which avoided the difficulty of spectral reflectance measurement of the concave mirror. Using the unit and a FEL lamp from NIST to calibrate the same ultraviolet radiometer

the results show that the bias between them is 3%

and the error of spectral irradiance calibration is 4.7% in range 200~300 nm.