We present theoretical discourses, demonstrating an unusual complex light-spin conversion in a plane rectangular graphene quantum dot (RGQD), deriving at the non-perturbative intensity of the pump wave-pulse at the process of high harmonic generation. Linearly polarized excitation leads to the emission of elliptically polarized harmonics. The ellipticity and the orientation of the polarization ellipse (the tilt angle of the q’th harmonic) are derived from the Stokes parameters computed from the intensity of the different polarization components. The numerical results demonstrate that the ellipticity of the harmonics generated in RGQD can be easily tuned by changing the tilt angle of the linearly polarized pump wave pulse, without the need for complex driving systems.