The wavelength calibration methods were investigated for shortwave infrared (SWIR) flat spectroradiometer. The shortwave infrared spectroradiometer includes two measurement units. The unit based on a concave holograph grating, has a flat focal plane along which a photodiode array (PDA) is placed. The wavelength calibration is performed in each respective band. In order to calibrate more accurately, calibration process is designed in accordance with the spectroradiometer characters. For wavelength calibration, the light source of fine spectral line becomes important. We select monochromator as light source which outputs light with spectral resolution of 1.5 nm in shortwave range. In this way, we preliminarily decide which pixel a spectral line is located at. Next, the position of the peak wavelength is determined by the center of gravity method. Then, the coefficients of polynomial curve are decided through data fitting. The calibration results show that in the range of 900～2400 nm the fitting error is less than 0.5 nm and calibration uncertainty better than 0.6 nm.