Abstract: Fraunhofer-Diffraction has not only theoretical but also practical meanings.The measurement of particle size is an important application of Fraunhofer-Diffraction.In order to precisely calculate the particle size,it is necessary to make the results of Fraunhofer-Diffraction unlimitedly to close to Mie solution.But the considerable difference between results of Fraunhofer-Diffraction and Mie solution exists,if the particle size parameter x,defined as x=πd/λ,closes by one.This paper proposed a more exact semiempirical formula to fit with the particle size distribution inversion.According to the semiempirical formula,the calculation results are exactly coincident with Mie's solution at small forward angles.Comparing with the approximation of Fymat A L and Mease K D,the formula gives much higher precise of measurement of particle size distribution for all the range,especially for minute particles.Also,this paper presented two methods to perform the particle size distribution inversion.