In this study, a compact rainbow refractometer is developed based on the global rainbow technology. This refractometer adopts a cage structure and a completely sealed design. The overall size of the refractometer, exhibiting a length, width, and height of 0.42, 0.42, and 0.15 m, respectively, is obviously smaller than those of the traditional refractometers. Further, the performance of the rainbow refractometer is validated by the experimental measurement of the refractive index of a single-component droplet, the concentration of a two-component droplet, and the droplet temperature. The accuracy of the refractive index measured using the rainbow refractometer is verified using a deionized water spray, and the measurement error is observed to be approximately 2×10 -4. The refractive indices of the water-ethanol droplets with volume fractions from 0 to 100% are measured and compared with the values in the literature, and the error sources are analyzed. The results denote that the developed compact refractometer exhibits a unique function of measuring the refractive indices of droplets; additionally, the compact refractometer possesses the advantages of small volume and high accuracy. Furthermore, it is applicable to industrial environments and exhibits a good application prospect in the measurement of spray fields.