Thermal　runaway　propagation　(TRP)　inside　lithium　iron　phosphate　(LFP)　batteries　is　an　important　part　of　TRP　process　of　the　module,　but　it　has　not　been　known　clearly.　This　work　experimentally　and　numerically　investigated　the　overheat-induced　thermal　runaway　(TR)　characteristics　of　243　Ah　LFP　battery　and　the　influence　mechanism　of　heating　power　on　TRP　within　the　battery.　A　series　of　TR　experiments　under　400,　700　and　1000　W　heating　power　were　conducted.　The　relationship　between　peak　jet　velocity　and　internal　average　TRP　velocity　(vTR)　was　revealed　for　the　first　time.　Results　show　safety　venting　and　TR　time　decreases　with　increasing　heating　power,　while　the　internal　TRP　time　prolongs.　Batteries　with　faster　internal　TRP　velocity　tend　to　own　higher　peak　jet　velocity.　The　relationship　between　vTR　and　heating　power　is　fitted　as　y　=　0.2421e_x/466.8843　+0.2351　based　on　simulation　result.　vTR　almost　decreases　linearly　with　heating　power　in　the　range　of　200-1000　W,　and　then　enters　a　plateau-like　descending　phase.　In　critical　TR　state,　the　width　of　TR　region　widens　as　heating　power　increases.　The　acceleration　of　vTR　induced　by　low　heating　power　is　mainly　attributed　to　a　better　pre-heating　effect　in　nonTR　region.　This　work　can　provide　important　guidance　for　the　safety　design　of　LIB　batteries.