Ferroelectric scandium-doped aluminum nitride (Al _1-x Sc _x N) has triggered tremendous research interests on account of the discovery of its superior ferroelectricity. Initial studies suggest that the Al _1-x Sc _x N may suffer from large leakage current and poor endurance, which may hinder its further thickness scaling and long-term reliability. In this work, we study the evolution of leakage current of the ferroelectric Al _0.7 Sc _0.3 N-based capacitor by bipolar and unipolar cycling tests. We observe that the leakage current dramatically increases only when the device undergoes bipolar cycling with the pulse amplitude larger than coercive electric field, and the Al0.7Sc0.3N film occurs grain fragmentation, which are caused by the polarization switching.