The reflectometer in spectral radiation standard and metrology beamline of Hefei National Synchrotron Radiation Laboratory mainly measures reflectance and transmittance of vavious reflecting coatings in X-ray and vacuum ultraviolet wavelength region. To ensure the accuracy and reliability of the measurement, the error sources of the reflectometer and their influence on measurement were investigated. Based on experimental data and theoretic calculation, it has been found that the accuracy of the measurement is significantly dependent on the stability of the synchrotron radiation, the detector and the fixing of the sample. Several typical forms of the radiation fluctuating, detector damage and sample fixing are summarized, and their influences as well as influence of light spot size on measurement are analysed quantitatively and qualitatively. Some effective measures are taken to control the measurement error within 2%.