  Based on first-principles calculations, we find novel valley-polarized
quantum anomalous Hall (VP-QAH) phases with a large gap-0.19 eV at an
appropriate buckled angle and tunable topological phase transitions driven by
the spontaneous magnetization within a half-hydrogenated Bi honeycomb
monolayer. Depending on the magnetization orientation, four different phases
can emerge, i.e., two VP-QAH phases, ferromagnetic insulating and metallic
states. When the magnetization is reversed from the +$\mathbf{z}$ to
-$\mathbf{z}$ directions, accompanying with a sign change in the Chern number
(from -1 to +1), the chiral edge state is moved from valley $K$ to $K'$. Our
findings provide a platform for designing dissipationless electronics and
valleytronics in a more robust manner through the tuning of the magnetization
orientation.
