Abstract: Based on rainbow Airy theory, the sensitivity of the main Airy peak position and Ripple frequency to the refractive index and diameter was studied. Combined with empirical mode decomposition for rainbow signal, Airy signal and Ripple component were extracted from rainbow distribution. According to the first-order rainbow distribution of droplet, an inversion algorithm was designed to calculate the refractive index and diameter of droplet simultaneously, which with powerful feature of noise immunity. The first-order rainbow distributions from water and ethanol droplets with different concentrations were measured in the experiment. The results show that the refractive index and diameter measurement accuracy of the droplet by rainbow-empirical mode decomposition method are 10⁻⁴ and 1% respectively.