Based　on　the　discrete　element　platform,　the　drained　triaxial　tests　of　gravel　soil　were　carried　out.For　these　tests,　the　method　of　staggered　spherical　particle　of　constructing　the　flexible　boundary　and　the　unfolding　method　of　applying　the　constant　confining　water　pressure　were　improved.By　using　particle　flow　code　(PFC3D),　three-dimensional　discrete　element　models　of　traditional　rigid　boundary　and　spherical　particle　flexible　boundary　were　established,　respectively,　for　numerical　simulations　of　drained　triaxial　tests　with　different　gravel　contents.Furthermore,　the　effects　of　boundary　conditions　on　the　simulation　of　the　deformation　and　development　of　gravel　soil　were　studied　from　the　perspectives　of　macro-failure　morphology　and　micro-mechanical　properties,　shear　zone　evolution,　and　development　of　force　chains　at　the　boundary.The　numerical　results　show　that:　in　the　cases　of　rigid　boundaries,　the　failure　of　the　specimen　occurs　earlier,　which　is　inconsistent　with　the　results　of　the　laboratory　test;　while　in　the　cases　of　flexible　boundaries,　the　failure　of　the　specimen　occurs　later,　conforming　well　with　the　laboratory　test;　With　the　flexible　boundaries,　the　uneven　bulging　deformation　at　failure　can　be　observed　in　the　middle　of　the　specimen,　and　the　inside　asymmetric　X-shaped　shear　band　is　formed,　while　with　the　rigid　boundaries,　the　deformation　of　the　specimen　appears　to　be　the　collapsed　failure.Based　on　the　comparison　of　distribution　and　evolution　of　the　boundary　force　chain　between　the　two　boundaries,　it　is　found　that　the　force　chain　of　the　flexible　boundary　is　evenly　distributed　with　more　balanced　strength;　for　the　rigid　boundary,　however,　stress　blind　zones　and　stress　concentration　may　occur　with　the　evolution　of　the　force　chain.Hence,　based　on　the　macro　and　micro　analyses,　it　is　recommended　to　apply　the　flexible　boundaries　rather　than　rigid　boundaries　for　better　numerical　simulations.　©　2021,　Editorial　Office　of　China　Civil　Engineering　Journal.　All　right　reserved.