Ocean internal wave is characterized with large amplitude, strong velocity, and short period and can cause a serious threat to the safety of offshore construction and underwater operations. The continental slope of the Northern South China Sea is one of the areas where internal solitary waves (ISW) and mesoscale eddies occur frequently. Therefore, the study on the influence of mesoscale eddy on internal solitary wave propagation is of great significance for understanding further the characteristics of internal solitary wave propagation during the anticyclonic eddy transit and improving the accuracy of the internal wave prediction model in the Northern South China Sea. For this purpose, the data obtained at a mooring station located in the west slope area of the Dongsha Islands in the Northern South China Sea during the transit of an anticyclonic mesoscale eddy in March 2017 are used for studying the influence of the mesoscale eddy on the internal solitary wave propagation. The results show that: ① Due to the influence of mesoscale eddy the average amplitude of the ISWs decreases by about 28.6%, for which the main reason is that the mesoscale eddy causes a depression of isotherm and then leads to an inhibitory action to the amplitude of ISW. Such an influence process can be described by the shallow thermocline theory; ② During the influence of mesoscale eddy the average wave speed of ISW increases from 1.26 m/s to 1.47 m/s and has an increasing range of about 16.7%, reflecting that the mesoscale eddy can strengthen the wave velocity of the ISW. This strengthening effect is mainly attributed to the change in background flow field caused by the edge flow field of the mesoscale eddy. However, the changes of temperature and salt fields caused by the mesoscale eddy have relatively little effect on the ISW speed.