Cryogen spray is an effective cooling technique used for laser treatment of Port Wine Stain. The cooling process involves complex droplet evaporation and strong convective heat and mass transfer, therefore a deep understanding of spray characteristics is essential in order 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 cryogen 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 consideration of droplet evaporation and convective effect with ambient air. The influence of variable physical properties of R134a is also considered. Based on the validation by simulating the cooling stage of a hanging water droplet, this model is used to predict the temperature variation of cryogen droplets in cryogen spray, which is in reasonable agreement with the experimental measurements. A parametric study of the influences of initial diameter and velocity on the droplet evaporation is then carried out. The results can be used to guide the Cryogen Spray Cooling of laser therapy.