The parameter inverse problem of rainbow measurement was studied. The mathematic model was built and a novel inverse algorithm was presented which could simultaneously measure particle size and refractive index. The new algorithm removes noise using the technique based on empirical mode decomposition (EMD). A feature point extraction technique was presented and an optimal point search algorithm based on Debye theory was proposed which can rapidly find the relatively exact inversion parameter. Simulation results show that when the signal-to-noise ratio (SNR) drops to 5 dB, the maximum inversion error of diameter is less than 10％ and error of refractive index is less than 0.1％. A laboratory experiment was also taken which concentrated on detection of a rainbow formed by a free-falling raindrop at different temperature. The droplet was illuminated by a continuum laser with the wavelength of 532 nm and power of 14 mW. The rainbow light formed by the droplet was collected by a lens with large diameter and received by a linear CCD placed in the focus of the lens. The experimental results also show that the method is feasible and has a relatively good precision.