The invention provides a positive electrode active material as well as a preparation method thereof, and an all-solid-state lithium battery. The positive electrode active material comprises nano-ferrous sulfide and a carbon layer coating nano-ferrous sulfide. The positive electrode active material provided by the invention, firstly, inhibits the volume expansion of ferrous sulfide during a redox reaction; secondly, the active material suppresses a shuttle effect of Li2S generated during the reaction to migrate to a negative electrode; thirdly, Li2S is an insulator which causes the ionic conductivity of the all-solid-state lithium battery to decrease, however, after coating with the carbon layer, the ionic conductivity of the all-solid-state lithium battery is improved due to the relativelyhigh ionic conductivity of the carbon layer; and fourthly, Li2S has no electrochemical activity, and Li2S is further electrochemically active after nano-ferrous sulfide is coated with the carbon layer, the polarization is reduced, and the reversibility of the battery is enhanced, thereby improving the cycle stability and battery specific capacity of the all-solid-state lithium battery comprisingthe positive electrode active material.