  Since the discovery of the fascinating properties in magic-angle graphene,
the exploration of moir\'e systems in other two-dimensional materials has
garnered significant attention and given rise to a field known as 'moir\'e
physics'. Within this realm, magnetic van der Waals heterostructure and the
magnetic proximity effect in moir\'e superlattices have also become subjects of
great interest. However, the spin-polarized transport property in this moir\'e
structures is still a problem to be explored. Here, we investigate the
spin-polarized transport properties in a moir\'e superlattices formed by a
two-dimensional ferromagnet CrI_3 stacked on a monolayer BAs, where the spin
degeneracy is lifted because of the magnetic proximity effect associated with
the moir\'e superlattices. We find that the conductance exhibits spin-resolved
miniband transport properties at a small twist angle because of the periodic
moir\'e superlattices. When the incident energy is in the spin-resolved
minigaps, the available states are spin polarized, thus providing a
spin-polarized current from the superlattice. Moreover, only a finite number of
moir\'e period is required to obtain a net spin polarization of 100\%. In
addition, the interlayer distance of the heterojunction is also moir\'e
modifiable, so a perpendicular electric field can be applied to modulate the
intensity and direction of the spin polarization. Our finding points to an
opportunity to realize spin functionalities in magnetic moir\'e superlattices.
