The　present　study　mainly　studies　the　effect　of　polydimethylsiloxane　(PDMS)　content　on　the　waterproofing　and　mechanical　properties　of　geopolymer　composites.　Firstly,　hydrophobic　modified　geopolymer　composites　(HMGC)　were　prepared　by　adding　PDMS　during　the　mixing　process.　Secondly,　the　surface　wettability　characteristics,　water　absorption,　uniaxial　compressive　and　tensile　properties　of　HM-GC　were　investigated.　The　effect　of　PDMS　content　on　the　waterproofing　and　mechanical　properties　was　further　discussed.　Finally,　considering　the　waterproofing　and　mechanical　properties,　the　optimal　PDMS　content　was　proposed.　The　results　showed　that　with　increasing　PDMS　content,　the　contact　angle　of　geopolymer　composites　rapidly　increase　at　first　and　then　stabilizes.　The　geopolymer　composites　with　4%　and　5%　PDMS　content　exhibit　overhydrophobic　surface　wettability.　In　addition,　the　water　absorption　gradually　decreases　with　increasing　PDMS　content,　indicating　an　improvement　in　the　waterproofing　ability.　The　incorporation　of　PDMS　can　enhance　the　compressive　properties　of　geopolymer　composites　while　reducing　the　tensile　properties.　Comprehensively　considering　the　waterproofing　and　mechanical　properties,　it　is　reasonable　to　select　4%　as　the　optimal　PDMS　content　used　in　practical　marine　engineering.