The invention discloses a rapid calculation method for mie scattering applied to dust particulate matter online monitoring. The method comprises the following steps that a particle scattering system is calculated, and a scattering coefficient table is formed; a particle size distribution mathematic model is built, and a reasonable initial value of a model parameter in an optimization algorithm is selected; particulate matter size distribution is subjected to inversion, and the particulate matter concentration is calculated. The particle theory scattering system is calculated in advance, a table look-up method is adopted for model parameter optimization calculation, repeated loop computation is avoided, and the calculation time is greatly shortened; a grid method is adopted in the algorithm for searching for a reasonable model parameter initial value for performing the first-time optimization algorithm, so that the global minimum value of a target function is accurately obtained; the model parameter initial value not measured at the first time is set as the model optimal parameter calculated the last time, iterations are reduced, and the operating speed is increased; the method has the advantages that the dust particulate matter size application range is large, and concentration change speed is large, and the method is used for a measuring system for performing real-time measurement of the dust particulate matter concentration.