The invention relates to the technical field of medical instruments and provides a blood oxygen saturation measurement method and device, an oxygen utilization rate measurement method and device and a medical instrument. The blood oxygen saturation measurement method includes the following steps that first, at least two optical signals, on which measured tissue acts, in different wavelengths are acquired; second, alternating-current amount and direct-current amount in a photoelectric plethysmography signal are respectively acquired; third, a blood oxygen saturation equation is determined according to a photon diffusion equation and extrapolation boundary conditions; fourth, blood oxygen saturation is acquired according to the ratio of the alternating-current amount to direct-current amount and the blood oxygen saturation equation. According to the blood oxygen saturation measurement method and device, the venous blood oxygen saturation is introduced, the blood oxygen saturation equation is determined according to the photon diffusion equation and the extrapolation boundary conditions, the source scientificity of blood oxygen saturation measurement is guaranteed, and accuracy of arterial blood oxygen saturation measurement is improved.