In　order　to　improve　heat　dissipation　performance　of　battery　pack　with　air-cooled　structure,　a　novel　stepped　divergence　plenum　in　Z-type　air-cooled　structure　is　proposed　in　a　prismatic　battery　pack.　Then　the　accuracy　and　effectiveness　of　computational　fluid　dynamics　(CFD)　model　are　verified　by　comparison　with　experimental　results.　Subsequently,　the　effects　of　the　total　height　of　all　steps　and　the　height　of　every　step　on　the　maximum　temperature,　temperature　difference　and　temperature　standard　deviation　of　battery　pack　with　air-cooled　structure　are　explored,　respectively.　Finally,　the　stepped　divergence　plenum　at　different　inlet　airflow　velocity　is　further　optimized　to　achieve　better　cooling　performance.　Numerical　results　indicate　that　1)　For　the　larger　total　height　of　the　steps,　the　stepped　divergence　plenum　is　suitable　for　Z-type　air-cooled　structure,　and　the　stepped　divergence　plenum　is　better　than　the　slanted　one　in　the　aspect　of　cooling　efficiency　while　the　latter　is　superior　to　the　former　for　the　smaller　total　height　of　the　steps　2)　Compared　with　the　slanted　divergence　plenum,　a　stepped　divergence　plenum　improves　the　heat　dissipation　performance　of　the　battery　for　Z-type　air-cooled　structure.　3)　The　height　of　every　step　and　inlet　airflow　velocity　plays　an　important　role　in　the　cooling　performance　of　the　battery.　The　optimization　results　are　h　=　0.125　(i.e,　Hopt　=　[1.39,　2.51,　2.51,　2.02,　2.02,　1.89,　1.76,　1.39,　1.51])　and　3　m/s,　and　the　corresponding　maximum　temperature,　temperature　difference　and　temperature　standard　deviation　of　the　battery　pack　are　respectively　reduced　about　34.65　%,　77.51　%　and　99.04　%　in　comparison　with　those　for　Z-type　model.　The　optimization　scheme　proposed　in　this　work　provides　a　design　guideline　to　improve　the　cooling　performance　of　the　air-cooled　structure.