Based on first-principles calculations, we study the effects of the chlorine atoms on electronic and magnetic properties of AlN nanosheets (NS). We find that both the bare and fully-chlorinated AlNNRs demonstrate semiconducting behavior, while the half-chlorination on surface Al sites leads to the semiconductor-ferromagnetism transition. More interestingly, the chlorination on surface Al sites in monolayer and bilayer AlNNSs demonstrates the half-metallic ferromagnetic (FM) behavior with 100% spin-polarized currents at the Fermi level, suitable for applications in spintronics at the nanoscale.