A method and device for detecting a blood flow rate are disclosed. The method comprises steps: providing a detection light source, and projecting a light of the detection light source onto a skin of a living body and reach a blood vessel using a photo sensor to receive a plurality of polarized light beams which are formed by the light of the detection light source reflected by expanding or contracting the blood vessel, wherein the photo sensor respectively generates a first PPG signal and a second PPG signal in blood vessel expansion and contraction using a microprocessor to acquire at least one first PPG signal and at least one second PPG signal, calculating an offset of received light beams and a time derivative thereof according to a sequence of the first PPG signal and the second PPG signal, and converting the offset and time derivative into a blood flow rate.
