Abstract: The Mie scattering theory is used to measure the particle size by light scattering. The calculation speed and precision of Mie scattering series are very important to the measurement results. Mie scattering series are calculated usually by forward recursion, backward recursion and link recursion for the different particle sizes and relative refractive indexes. The improved method based on the above mentioned three methods is reported, in which the initial value of Mie scattering series is calculated by continuous fractions and the others are calculated by backward recursion. The data are saved in the array data type and the program uses the recurrent method while the algorithm is realized in Matlab. The comparison of the results indicates that the calculation time is very short and the results are hard to overflow. Numerical calculation shows that the algorithm is efficient, reliable and robust in extremely wide range of particle size and refractive index.