The transmission rates of the next generation intelligent optical networks are high, and the use of dense wavelength division multiplexing and a reconfigurable optical add-drop multiplexer makes the traditional optical signal-to-noise ratio (OSNR) monitoring techniques unsuitable. Hence, we investigated the OSNR monitoring method based on the nonlinear effect of four-wave mixing (FWM), which applies to the next generation intelligent optical networks. Due to FWM in a highly nonlinear fiber, a converted wave is generated and its power is dependent on the OSNR of the input signal. Two scenarios are studied, single-pump and orthogonal-polarized dual-pump. Under the condition of the given fiber length and the appropriate pump power, the monitoring ranges of OSNR could be extended to 23 and 30 dB for single-pump and orthogonal-polarized dual-pump, respectively.