The invention relates to the field of optical communication, in particular to an optical signal to noise ratio detection method and an optical signal to noise ratio detection device. The device comprises a polarization state switching module, a polarization beam splitter, a first photoelectric conversion module, a second photoelectric conversion module, a power comparison module and an optical signal to noise ratio (OSNR) computing module, wherein the polarization state switching module performs preset switching on the polarization state of to-be-detected light input into the polarization state switching module according to an indication signal generated by the power comparison module, so that an average power difference between two optical signals output by the polarization beam splitter meets preset requirements; and when the average power difference between the two optical signals meets the preset requirements, the OSNR computing module performs signal equalization on electric signals obtained by the photoelectric conversion of the two optical signals, and computes the OSNR of the to-be-detected light based on the equalized signals. By the OSNR detection scheme provided by the embodiment of the invention, a polarization controller is not required, and the OSNR of the to-be-detected light can be detected rapidly and highly efficiently with low cost.