Cryogen spray is an effective cooling technique for the laser treatment of port wine stain (PWS),which can protect the epidermis from the thermal injuries safely. However,cryogen R134a cannot supply enough cooling capacity for the laser surgery in patients with darker skin because of the absorption of laser energy by melanin. Cryogen R404a are expected to protect the darker skin well because of its lower boiling point. A theoretical model is proposed to describe the evaporation process of single droplet and its validation is confirmed by the agreement between numerical simulation with experimental result. With this model,the cooling effect of R404a droplet is compared with R134a droplet and it is found that R404a droplet can reach lower temperature. Then the influences of initial diameter and ambient mass faction of cryogen on the evaporation characteristics of R404a droplet are analyzed and the result is useful to guide the cryogen spray cooling for laser therapy of PWS patients with darker skin.