The　behavior　of　square　GFRP　tube　beams　filled　with　seawater　and　sea　sand　concrete　under　the　four-point　test　was　studied　with　48　specimens.　The　main　test　parameters　of　this　study　included　cross　section　size,　GFRP　tube　thickness,　and　sea　sand　replacement　ratio.　The　failure　process　of　the　specimen　during　the　four-point　test　was　studied.　The　reslut　indicated　that　the　effect　of　GFRP　tube　strengthening　core　concrete　on　the　failure　process　was　significant.　There　was　no　apparent　elastoplastic　stage　before　the　specimen　failed,　thus　the　failure　mode　was　brittle　under　test,　which　could　be　found　from　the　load-displacement　curve.　In　addition,　the　effects　of　these　test　parameters　on　the　ultimate　capacity,　flexural　rigidity,　and　ductility　were　analyzed　in　this　study.　The　test　results　indicated　that　the　effects　of　GFRP　tube　thickness　on　the　ultimate　capacity　and　flexural　rigidity　were　significant.　The　replacement　of　normal　concrete　with　sea　sand　concrete　had　no　obvious　negative　effect　on　the　mechanical　properties　of　the　GFRP　tube　beams　filled　with　concrete.　Furthermore,　the　calculated　formula　of　the　ultimate　capacity　of　the　GFRP　tube　beams　filled　with　seawater　and　sea　sand　concrete　was　proposed　in　this　study.　The　calculated　results　were　compared　with　the　test　results,　which　indicated　that　the　calculated　formula　was　greatly　agreed　with　the　test.　©　2022