Abstract: The KdV and eKdV theories are used to describe the waveform and velocity field of internal solitary waves. The results are connected with the Morison equation to establish a theoretical horizontal force model for classical Spar platforms. The calculation parameters of internal solitary waves are set by referring to the real characteristics of the internal solitary waves in the South China Sea. The internal solitary wave force on the classical Spar platform Hoover is calculated by using KdV and eKdV methods and the results are compared. It can be seen from the results that the horizontal force calculated by the eKdV theory is similar to that of the KdV theory, but the waveform of the eKdV theory is gentler, which has longer acting time but slightly smaller wave force.