Simultaneous measurement of the concentration and size of spray droplets plays an important role in understanding sprays. The global rainbow technique is applied to measure the concentration and size of bi-component droplets. Based on the modified Nussenzweig theory, the refractive index and size distribution are optimally evaluated, and then the concentration is retrieved with prior relationship between the refractive index and concentration. The inverse algorithm is verified by the simulated global rainbow signals; results shows that it can accurately retrieve the refractive index and size distribution of the droplets with an unimodal or bimodal particle size distribution. Sprays of water-ethanol solution with volume concentration from 0% to 100% are measured. Results show that the measured refractive indices agree well with the exact values, and the size distribution is stable. The global rainbow technique shows great potential in concentration measurement of spray droplets.