In　terms　of　support　form　for　foundation　pit,　the　most　common　calculation　method　of　internal　force　and　displacement　of　retaining　piles　(or　enclosure　wall)　plus　internal　bracings　is　incremental　method,　which　is　based　on　the　theory　of　beam　on　elastic　foundation.　Reasonable　calculation　of　load　increment　is　the　key　to　implement　this　method.　At　present,　the　load　increment　of　this　method　mainly　consists　of　earth　pressure　increment　and　reaction　increment　released　by　the　excavated　soil　elastic　resistance.　Based　on　the　basic　principle　of　the　method　of　beam　on　elastic　foundation　and　incremental　method,　combined　with　excavation　construction　process,　the　following　conclusions　were　drawn　through　the　comprehensive　analysis.　In　addition　to　the　above　two　parts,　the　load　increment　also　included　reaction　increment　by　soil　elastic　resistance　released,　which　was　partially　caused　by　the　decrease　of　the　excavation　face　and　the　reduction　of　the　horizontal　resistance　coefficient.　The　reaction　increment　of　random　point　of　the　retaining　piles　(or　enclosure　wall)　in　this　part　that　below　the　excavation　face　was　equal　to　the　product　of　the　lateral　displacement　of　this　point　in　the　previous　working　condition　and　the　proportional　coefficient　of　the　soil　horizontal　resistance　coefficient　(m),　and　the　excavated　soil　thickness　in　the　current　working　condition.　The　comparative　analysis　of　theoretical　calculation　results　and　measured　results　of　engineering　examples　show　that:　compared　with　the　traditional　incremental　method,　utilizing　the　load　increment　mentioned　in　this　paper　to　do　load　increment　calculation　(the　improved　incremental　method)　yields　a　value　of　lateral　displacement　of　retaining　piles　(or　enclosure　wall)　that　is　much　closer　to　the　measured　value.　The　calculation　result　is　more　in　line　with　the　engineering　practice.　©　2018,　Science　Press.　All　right　reserved.