To measure oxygen content in port wine stain (PWS) skin using diffuse reflectance spectroscopy and monitor the change in oxygen content during photodynamic therapy (PDT), the diffuse reflectance spectra of port wine stain skin were measured using the USB2000 miniature fiber optic spectrometer immediately 10, 20, 30 and 40 min after treatment began, then oxygen content at each time point was analyzed based on the difference of absorption spectra between Hb and HbO2, and the change trend can be observed. The diffuse reflectance spectra of 14 PWS patients were monitored during treatment, and the change trend of oxygen content were analyzed. Results showed that before treatment the oxygen content varies between patients with different type of PWS. During PDT, there was no obvious change in oxygen content for skin of most P4 and P5 PWS, while obvious decline was observed in oxygen content for P6 PWS. The current diffuse reflectance spectroscopy can be used to monitor oxygen content in PWS skin qualitatively and observe the change trend. To get more precise oxygen content in PWS skin, more work is needed to improve the sensitivity of diffuse reflectance spectroscopy and a spectra algorithm should be established.