For the measurement of a single drop in spray, the Debye series for scattering of a Gaussian beam by a spherical drop is used to study the light intensity distribution in the primary rainbow region and the influence of the Gaussian beam on the peak position of the intensity distribution. According to the intensity distribution calculated by the Debye series, the refractive index and size of drop are inversed, which prove that the Gaussian beam scattering could be used for drop measurements. And then, generalized Lorenz-Mie theory is used to compute the total light intensity which is used to inverse the refractive index and size of drop. And the influence of Gaussian beam position on the inversion of drop information is also investigated. For a Gaussian beam centered on the Descartes ray position, the absolute error of the refractive index is smaller than 2.38×10-4 and the absolute value of relative error of drop radius is smaller than 3.13% for drops with a radius between 200 and 1 000 μm. Compared with the rainbow technique using parallel beam, it possible to obtain a measurement area with high intensity and appropriate volume by using Gaussian beam. Therefore, the simultaneous occurrence of many drops in measurement area can be effectively avoided. And we could reduce the influence of multiple scattering between drops and increase the signal intensity.