The invention discloses a polarization interference-based in-band optical signal-to-noise ratio detection method and device. The method comprises the following steps of: dividing an input optical signal into two beams, of which one beam is used for measuring total power of the optical signal, and the other beam is used for measuring noise power in the optical signal; performing polarization beam splitting on the optical signal to obtain two branches which are perpendicular to each other in the polarization direction; adding time delay in one path of the two branches; combining the optical signals of the two branches together to obtain an optical signal with a polarization state varying along with frequency; adjusting a polarization angle of a polarizer to realize complete destructive interference of some frequency points so as to obtain an optical spectrum comprising noise power information; and calculating the optical signal-to-noise ratio according to the obtained total power of the optical signal and the noise power. According to the method and the device, the OSNR (Optical Signal to Noise Ratio) of the signal in a dynamic optical network and a high-speed optical fiber communication system can be detected, the detection range is 9-35 dB, the detection is not affected by the signal polarization state, optical fiber dispersion and polarization mode dispersion, the method and the device are transparent to a signal modulation format and a transmission rate, and the defects of the conventional OSNR detection method are overcome.