Cryogen spray cooling (CSC) is an effective cooling technique used for laser treatment of port wine stain (PWS). The cooling process involves complex droplet evaporation, strong convective heat and mass transfer, therefore a deep understanding of spray characteristics is essential to optimize the nozzle design and improve the cooling efficiency of the spray. This paper presents a theoretical model to describe the equilibrium evaporation process of a single droplet in cryogen spray. The model considers mass transfer through mass transfer number method, the momentum transfer by selecting the suitable empirical correlation of drag coefficient, and heat transfer by taking into account the droplet evaporation and convective effect with ambient air. Through simulating the cooling stage of a hanging water droplet and the temperature variation of cryogen droplets in cryogen spray, the model is validated by the reasonable agreement with the experimental measurements. Then a parametric study of the influences of initial diameter and velocity on the droplet evaporation is carried out, which states that an effective analysis method can be provided by the proposed model to guide the CSC of laser therapy.