Abstract: Most laser particle sizer based on light scattering adopt such a structure in which the sample cell is put behind the Fourier lens.The angle range of scattered light received by a ring of the concentric-ring-shipped photoelectric detector is determined according to the Fourier optics.In accordance with such determaination,the reason of resulting error under large-angle scattering has been analyzed,the magnitude of the errors has been calculated,and the effect on particle sizing has been discussed.The formula,which is proper to determining the angle range of scattered light received by every detecting ring,have been derived.