We investigate the fractional spin ferroelectric (FSFE) in magnetic sliding ferroelectrics (SFEs), where ferroelectric switching is characterized not only by the reversal of the out-of-plane electric polarization but also by a variation of fractional in-plane spin electronic polarization. We show that interlayer sliding in FSFEs can naturally lead to a symmetry-protected pure spin current, termed the sliding spin current here. The underlying mechanism is that, during switching, the contributions of valence electrons and ions to the in-plane charge transfer cancel each other, whereas the in-plane spin transfer, which stems solely from valence electrons, persists, leading to a pure spin current. We demonstrate our ideas in various material candidates, including H-stacked bilayer CrI₃, whose few-layer form has been experimentally confirmed to be a magnetic SFE, and R-stacked bilayers 2H-VX₂ (X= S, Se, Te), which have been experimentally synthesised. For a typical switching time of about 1 ns, the estimated spin-current densities for bilayer CrI₃ and VX₂ reach 10⁹ (ℏ/2e)A/m² and 10⁸ (ℏ/2e)A/m², respectively. This means that by applying a periodic out-of-plane electric field, a significant alternating spin current can be generated in magnetic SFEs. Thus, our findings propose a compelling new mechanism for the all-electrical generation of pure spin current, and predict concrete realistic materials for experimental verification.