Global rainbow technique (GRT) is an effective method for the simultaneous measurement of the size distribution and refractive index of the spray droplets. However, there are some shortcomings in the existing algorithm. An improved reversion algorithm of GRT is proposed based on the least square method and optimization process. The features are extracted from the intensity distribution of the captured global rainbow to estimate the initial value of the size distribution and refractive index of the droplets, so as to build the linear equations. The linear equations are solved by the least square method to get the particle size distribution, and global rainbow is reconstituted according to the results. The refractive index and size distribution of the droplets are optimized according to the angular deviation between the reconstructed and captured rainbows. Both numerical simulations and experiments are carried out to evaluate the feasibility of the improved algorithm. The results show that the error of the inversion result of refractive index is of the order of magnitude 10-4, and the error of the mean diameter is better than 3 μm, and the reverse grain diameter is in accordance with the true value.