In this study, through changing the shape of YIG slab, we demonstrated the strong influence of the demagnetizing field on magnetic properties in YIG slab and various spin current transports in Pt/YIG, including the spin Seebeck effect, spin Hall effect, spin Hall magnetoresistance, and planar Hall resistance. The thickness- and width-dependent unusual plateau behaviors of the spin-dependent electrical and thermal transports as well as magneto-optical measurements were manifested from the shape of the YIG sample and closely related to the calculated effective demagnetizing factors. Furthermore, we directly visualized the evolution of the magnetic domains with an abrupt 90° magnetic rotation through magnetic force microscopy and we further corroborated it by the 90° resistance phase shift in the angular-dependent planar Hall effect. We provide sufficient evidences to demonstrate the spin current transport is not only sensitive to the surface magnetic structure but also reflects the shape of bulk for a magnetic material.