For the inversion algorithm of field particle size distribution, a new method was proposed with the improved R-R distribution in the dependent model algorithm．Multi-group size parameters (X) and distribution parameters (N) were set simultaneously, and then their target functions (fitting errors) were inversed by multi-threads technology. When the minimum fitting errors were found, it was considered that the particle size distribution function was an R-R distribution with corresponding X and N parameters. The experiments of field laser particle size analyzer illustrated that it took no more than one minute averagely in each inversion if using 5 threads. The method has advantages of simplicity，high accuracy, repeatability and rapidity，and can meet the needs of field particle size real-time measurement.