To reduce noise in the inversion for diffraction measurement of particle size distribution from scattering spectrum, an improved method was proposed. A regulate function was induced in the Chin-Shrine integral transform that led to the noise disappearing and didn’t affect the peak of inversion spectrum. The improved method was tested using linear CCD detector array. The result showed that the method largely improved previous results because of using the regulate function, and it is feasible and effective. The authors also explained that selecting the minimum and maximum of sampling angle by way of the different focal length reduced the different trouble of inversion spectrum. Using suitable focal length achieved good results in actual measurements.