The in-orbit synchronous calibration of a spaceborne optical sensor needs to use the ground synchronous spectral measurement as the data support. The accurate surface reflectance is one of the important prerequisites for obtaining high-precision calibration results. The traditional field ground reflectance measurement methods use a single spectrometer to measure a target and a reference panel in turn. In the case of dramatic irradiation change, the traditional measurement methods can not keep the radiation condition of the target and reference panel to be consistent. By using a dual-spectrometer method to measure the reflectance-time series of the ground objects in the field, the problem of inconsistent radiation condition occurred possibly in the traditional methods is avoided. By comparing the measurement result of the dual-spectrometer method with that of the single-spectrometer method, the correlation coefficient is up to 0.998. Moreover, the reflectance curve obtained by the dual-spectrometer method is closer to the actual spectrum. Thus, the feasibility of the dual-spectrometer method is verified.