The present invention discloses a method and an apparatus for detecting an optical signal-to-noise ratio, a node device, and a network system. The method includes: receiving a detected optical signal carrying amplified spontaneous emission ASE noise; detecting a first alternating current component and a first direct current component of the detected optical signal; acquiring first modulation information of the detected optical signal; acquiring first correction information corresponding to the first modulation information according to the first modulation information; and determining an optical signal-to-noise ratio OSNR of the detected optical signal according to the first alternating current component, the first direct current component, and the first correction information.