The invention relates to a skin disease quantitative evaluation device which is specifically used for achieving the quantitative evaluation on a skin disease such as nevus flammeus through the spectrum absorption rate. According to the skin disease quantitative evaluation device, a spatial light modulator is used for regulating a multi-spectrum light source to form a structured light image on the surface of a patient and synchronously demodulating a coded image, a three-dimensional structured light image, under the multi-spectrum light source, of the patient is obtained, a three-dimensional model of an affected area is obtained, meanwhile, the corrected multi-spectrum absorption difference of an affected part and a normal part is obtained according to the incident angle and light distribution difference of light rays in the three-dimensional model, and fine grading of the disease is completed under processing of a computer.