Solar simulator is a key instrument for photovoltaic field, which aims to act the role of natural sunlight irradiance indoor, and we should identify how similar they are in quantity. The critical factor of similarity lies in its spectral irradiance, because of solar cells’ wavelength-dependent spectral responsivity, spectral mismatch of solar simulator and sunlight can induce large errors during characteristic parameters measurement. In this article, a method for measuring solar simulator’s spectral irradiance was proposed along with its uncertainty analysis. A calibrated fiber optic spectrometer was employed here for spectral measurement, which was used for calibrating various kinds of solar simulators manufactured with different mechanisms. Considering three main sources of measurement uncertainty, that is, the declared uncertainty of the calibrated spectrometer (u1), cosine correction (u2) and repeatability of measurement (u3), we estimated its combined expanded uncertainty is U = 6.2% (with coverage factor k = 2). Also, we have made a comparison of our spectral measurement results with methods traceable to other country’s national institute of metrology, such as NIST traceable. This work is significant for the performance calibration and classification of solar simulators, so that plays a great role in solar energy industry.