In advanced high-speed fiber optic communication systems, due to the introduction of dense wavelength division multiplexing technology, the signal spectral interval is getting narrower and narrower, and the traditional out-of-band Optical Signal-to-Noise Ratio (OSNR) monitoring technology is no longer accurate. Therefore, further study is required in the low-cost in-band OSNR monitoring scheme. A Deep Neural Network (DNN) link OSNR monitoring scheme for Intensity-Modulation and Direct Detection (IMDD) system is proposed. We used a 5-layer DNN trained from 550 000 datasets to successfully estimate the OSNR of the 2 GBaud On-Off Key (OOK) signal in the range of 5 to 15 dB, and the Mean Absolute Error (MAE) is less than 0.8 dB.