This　paper　experimentally　investigated　the　effect　of　low　vent　burst　pressure　on　the　overpressure　buildup　and　flame　evolution　during　the　vented　methane-air　deflagrations　in　a　1　m3　rectangular　vessel.　CFD　software　FLACS　was　assessed　in　simulation　of　vented　methane-air　explosions　against　the　experiment.　Experimental　results　demonstrate　that　four　overpressure　peaks　of　Popen,　Pout,　Pext　and　Pacc　from　the　vent　failure,　flame　venting,　external　explosion　and　acoustically　enhanced　combustion,　respectively,　were　observed　except　for　the　free　venting　case,　in　which　only　two　overpressure　peaks　of　Pout　and　Pext　were　monitored.　The　flame　behaviors　corresponding　to　different　overpressure　peaks　were　monitored　especially　at　Pacc,　where　an　abrupt　increase　in　flame　luminosity　was　observed　on　the　bottom　of　the　vessel　due　to　the　combustion　of　pockets　of　flammable　mixture.　Popen　and　Pacc　increase　with　the　vent　burst　pressure　while　Pout　and　Pext　are　nearly　independent　of　the　vent　burst　pressure.　Double　flame　accelerations　were　observed　owing　to　the　vent　rupture　and　flame　venting.　The　phenomena　of　Helmholtz　oscillation　and　the　bulk　motion　of　flame　bubble　always　appeared　after　the　vent　rupture.　The　predicted　overpressure　agrees　reasonably　well　with　the　experimental　data.　The　discrepancy　of　the　predicted　and　measured　maximum　peak　overpressures　reduces　with　increasing　vent　burst　pressure.　The　predicted　maximum　peak　overpressure　and　external　flame　length　are　slightly　larger　than　those　in　experiment,　which　would　facilitate　the　safe　design　in　explosion　venting　and　offer　a　better　guidance　in　risk　assessment.　The　simulation　results　also　show　that　two　overpressure　peaks　were　recorded　resulting　from　the　pressure　wave　propagating　above　the　vessel　and　the　external　explosion,　respectively.　©　2020　Institution　of　Chemical　Engineers