The filter-fluorescer method is an important X-ray diagnostic technique. For 14×107—16×107 K blackbody radiation, a four channel spectrometer consisting of filter-fluorescer combinations of V-Ti, Co-Fe, Cu-Ni and Zn-Cu was designed, and the corresponding supplementary channels were supplied for each primary channel. The calculation results show that the measurement of supplementary channel which has the same configuration as that of primary is not accurate, the postfilters of supplementary channels should be reconfigured to make an proper background measurement.