Double crystal diffractometer applied to measure SAXS is very satisfactory, since it has high resolving power and relatively small measure area. But for a long time it has not been used widely, because it has comparatively strong background which may cover the signal of SAXS. We point out that this background results from two kinds of its sources, one is the fluorescence come from the second crystal; the other is the effect of tail part of the rocking curye of crystal used. The contribution of the first source can be subtracted by a pulse height analyser. We present here a method that can subtract the effect of the tail part of rocking curve. Experiment domonstrates that this method is satisfactory.