Jet　fire　is　a　typical　behavior　of　Li-ion　batteries　during　thermal　runaway　(TR),　which　is　one　of　the　main　factors　causing　fire　accidents　in　battery　systems,　but　its　dynamic　dominant　mechanism　is　still　unclear.　Therefore,　this　paper　experimentally　investigated　the　dynamic　evolution　of　jet　fire　of　lithium　iron　phosphate　(LFP)　and　lithium　nickel‑cobalt-manganate　(NCM)　batteries　with　the　same　capacity.　A　novel　method　based　on　image　recognition　algorithm　and　nonlinear　fitting　was　proposed　to　reveal　the　dominant　mechanism　of　jet　fire　and　to　further　divide　its　process　into　stages.　Besides,　the　effect　of　the　state　of　charge　(SOC)on　the　dynamics　of　jet　fire　was　quantified.　The　results　show　the　jet　fire　of　50%SOC　NCM　battery　goes　through　five　stages　successively:　buoyancy　dominant,　buoyancy-momentum　co-dominant,　momentum　dominant,　buoyancy-　momentum　co-dominant　and　buoyancy　dominant,　which　is　one　more　momentum　dominant　stage　than　that　of　50　%　SOC　LFP　batteries.　For　both　50　%　SOC　NCM　and　LFP　batteries,　the　time　ratio　of　buoyancy-momentum　co-dominant　during　TR　are　the　highest,　reaching　85.1　%　and　79.1　%,　respectively.　The　outlet　kinetic　energy　of　50%SOC　LFP　battery　is　greater　than　that　of　buoyancy　work　during　TR　process,　while　the　proportion　of　time　that　the　kinetic　energy　on　fire　is　greater　than　that　of　buoyancy　work　in　NCM　battery　is　47.49　%,　and　the　time　ratio　of　pure　momentum-dominated　is　5.98　%.　Furthermore,　the　effect　of　gas　outlet　momentum　on　the　fire　increases　with　the　increase　of　SOC.　This　paper　can　provide　a　theoretical　direction　for　reducing　the　fire　hazard　of　LIBs.　©　2025