Controlling spin dynamics at ultrafast timescales is crucial for next-generation spintronic and quantum devices, yet achieving precise control and understanding the underlying microscopic mechanisms remain challenging. Here, we demonstrate that ferroelectric polarization in $\mathrm{Cr}{\mathrm{I}}_3/\alpha -\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ heterostructures serves as a multifunctional switch that selectively regulates spin-relaxation behavior. Using time-dependent density functional theory combined with nonadiabatic molecular dynamics, we show that femtosecond laser excitation induces similar ultrafast demagnetization in both polarization states, while the subsequent relaxation dynamics of photoexcited carriers diverge dramatically. Polarization-controlled interfacial electric fields and band alignments steer photoexcited electrons along two distinct pathways: a stepwise interlayer transfer in the polarization-up ($P\uparrow$) configuration and a direct intralayer spin-flip in the polarization-down ($P\downarrow$) configuration. These bifurcated dynamics originate from the ferroelectric-tuned interplay between spin-orbit coupling and electron-phonon interactions. These findings establish a microscopic link between ferroelectric control, spin-orbital coupling, electron-phonon coupling, and ultrafast spin dynamics, offering a route for all-optical, non-volatile manipulation of spins in two-dimensional heterostructures.

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• Electronic structure
• Laser-induced demagnetization
• Spin-orbit coupling