Abstract: Investigation on the propagation and evolution of internal solitary waves (ISWs) in the continental slope area of the South China Sea (SCS) is crucial for understanding the mechanisms of internal solitary waves and ensuring the safety of offshore oil and gas engineering. Observational data in the northeastern continental slope area of the SCS are used in this study to analyze the propagation speed of internal solitary waves. The theoretical propagation speeds based on the Korteweg-de Vries (KdV) equation are estimated and compared with the observations. Results show that the first-mode concave internal solitary wave is dominant in the northeastern continental slope area of the SCS. A total of 370 first-mode ISWs were identified and tracked at all the stations, with propagation direction ranging from 274° to 322°. The mean propagation speed varies from 0.78 to 1.52 m/s, showing seasonality with higher speed in summer and autumn and lower speed in winter and spring. The theoretical propagation speed calculated from the KdV equation ranges from 0.61 to 1.43 m/s, which explains 58.1% to 99.9% of the observed propagation speed. The consistency between the KdV theory and the observations varies in different seasons, with median agreement values for spring, summer, autumn, and winter being 91.2%, 91.5%, 86.3%, and 85.9%, respectively. This study assessed the applicability of the classical KdV theory in explaining ISWs propagation speeds in the continental slope area of the SCS. The findings are helpful for accurate prediction of the ISWs.