Compared　with　the　conventional　drainage　strengthening　techniques,　the　precast　concrete　segment　assembly　strengthening　method　(PCSAM)　is　regarded　as　a　fast,　more　economical,　and　traffic-friendly　underwater　strengthening　method　for　damaged　bridge　piers　and　piles,　as　the　drainage　procedure　can　be　omitted.　However,　this　method　still　has　some　disadvantages,　such　as　strength　loss　of　the　filling　material,　debonding　of　the　interface　due　to　shrinkage　of　the　filling　material,　poor　connection　effects,　and　poor　durability　of　the　segment　sleeves.　To　solve　these　problems,　the　PCSAM　is　improved　in　this　study　by　using　self-stressed　anti-washout　concrete　(SSAWC)　as　the　filling　material　and　by　developing　a　lining　concrete　segment　sleeve　(LCSS)　by　referring　to　the　design　theory　for　shield　lining　segments.　Six　specimens　are　designed　and　prepared　with　consideration　of　the　influential　factors,　such　as　the　self-stress,　thickness　of　the　filled　concrete,　and　concrete　strength　of　the　LCSS,　then　the　monotonic　axial　compression　test　is　carried　out　to　investigate　the　improvements　in　the　axial　compression　properties　of　the　specimens.　Accordingly,　extended　parametric　analyses　are　performed　based　on　the　established　numerical　models.　Finally,　the　calculation　formula　for　the　bearing　capacity　is　proposed　based　on　the　analysis　results.　The　results　indicate　that　the　SSAWC　can　provide　initial　confining　compressive　stress　in　the　core　region　of　the　piers,　in　addition　to　increasing　the　bearing　capacity　and　ductility　of　the　specimens.　The　improved　LCSS　segment　connection　is　more　reliable　and　increases　the　strengthening　efficiency.　The　influence　of　self-stress　on　the　bearing　capacity　of　the　specimens　is　cubic　and　the　influence　of　the　filled　concrete　strength　on　the　bearing　capacity　of　the　specimens　is　nonlinear.　The　calculation　formula　for　predicting　the　bearing　capacity　of　axially　compressed　columns　possesses　good　applicability　and　can　be　used　as　a　reference　for　practical　engineering.　©　2021　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.