This application provides a positive active material and a preparation method thereof, an electrochemical battery, a battery module, a battery pack, and an apparatus. The positive active material includes an inner core and a coating layer, where the coating layer coats a surface of the inner core. The inner core is selected from a ternary material with a molecular formula of Li1+a[NixCoyMnzMbM′c]O2−dYd, where distribution of each of the doping elements M, M′, and Y in the inner core meets the following condition: there is a reduced mass concentration gradient from an outer side of the inner core to a center of the inner core. The positive active material herein features high gram capacity, high structural stability, and high thermal stability, so that the electrochemical battery has excellent cycle performance and storage performance and high initial discharge gram capacity.