Mie theory and design of a probe of cloud droplet based on this theory are introduced in this paper. A 685 nm diode laser is used to illuminate the cloud droplets in the sensitive area in the probe. The forward-scattering part and backward-scattering part are included in the optical receiving system. The former collects energy between the solid angle of 4°～12°, and the latter collects energy between 126°～134°. The size of the cloud droplet can be obtained by the data from the energy of the forward-scattering. When the size of the cloud droplet is known, the phase state of the cloud droplet can be inferred from the ratio of forward-scattering to backward-scattering. In the end, the intensities of forward-scattering varied with sizes of cloud droplets and the ratios of forward-scattering to back-scattering varied with the refractive indexes are presented by using the Mie theory.