The　modified　suction　caisson　(MSC)　is　a　novel　type　of　foundation　for　ocean　engineering,　consisting　of　a　short　external　closed-top　cylinder-shaped　structure　surrounding　the　upper　part　of　the　regular　suction　caisson　(RSC).　The　MSC　can　provide　larger　lateral　bearing　capacity　and　limit　the　deflection　compared　with　the　RSC.　Therefore,　the　MSC　can　be　much　more　appropriate　to　use　as　an　offshore　wind　turbine　foundation.　Model　tests　on　the　MSC　in　saturated　sand　subjected　to　monotonic　lateral　loading　were　carried　out　to　investigate　the　effects　of　external　structure　sizes　on　the　sand　surface　deformation　and　the　earth　pressure　distribution　along　the　embedded　depth.　Test　results　show　that　the　deformation　range　of　the　sand　surface　increases　with　the　increasing　width　and　length　of　the　external　structure.　The　magnitude　of　sand　upheaval　around　the　MSC　is　smaller　than　that　of　the　RSC　and　the　sand　upheaval　value　around　the　MSC　in　the　loading　direction　decreases　with　the　increasing　external　structure　dimensions.　The　net　earth　pressure　in　the　loading　direction　acting　on　the　internal　compartment　of　the　MSC　is　smaller　than　that　of　the　RSC　at　the　same　embedded　depth.　The　maximum　net　earth　pressure　acting　on　the　external　structure　outer　wall　in　the　loading　direction　is　larger　than　that　of　the　internal　compartment,　indicating　that　a　considerable　amount　of　the　lateral　load　and　moment　is　resisted　by　the　external　skirt　structure.