We propose a novel and simple method for obtaining the giant longitudinal spin Hall effect (SHE) of the reflected light beam when the elliptically polarized light (instead of the linearly polarized light) at the telecommunication wavelength is obliquely incident on a prism–sodium interface excited by surface plasmon resonance. By introducing the spatially averaged Stokes parameter  for a non-uniformly polarized reflected light field, understanding the generation mechanism of the giant longitudinal SHE from a new perspective is realized. The giant longitudinal SHE under the elliptically polarized light reaches 60.28 μm by the optimal parameter setup, and the spin splitting direction of the SHE can be switched by adjusting the amplitude ratio angle and phase difference of the incident elliptically polarized light. These findings open the way for the precise measurement of the ellipticity of the elliptically polarized light and the design of novel fiber-optic devices.