The temperature and salinity analysis from three operational forecasting systems (PSY4 of Mercator Océan International of France, GOFS3.1 of HYCOM Consortium of the United States, and CGOFS of the National Center for Marine Environmental Forecast of China) were evaluated with the observation data by "Haiyi" underwater glider in the northern South China Sea from July 12, 2019 to September 20, 2019. In general, the three forecasting systems can well reproduce the distribution and variability of temperature and salinity. The RMSE of temperature and salinity of PSY4 system are 0.45 ℃ and 0.056, respectively, which are smaller than those of the other two forecasting systems. Evaluation also shows that the temperature produced by the PSY4 and CGOFS forecasting systems presents warm bias, while that by the GOFS3.1 presents cold biases, and the salinity of all the three systems exhibits negative biases. In addition, the salinity in August 2019 produced by the CGOFS shows a large RMSE and significant cold deviation, which might be due to the large precipitation in its atmospheric forcing field.