The　reaction　between　gypsum　rock　and　water　results　in　swelling　deformation　and　swelling　pressure.　Swelling　deformation　and　swelling　pressure　cause　damage　to　underground　engineering　such　as　tunnels.　It　is　of　significance　to　study　the　swelling　characteristics　of　gypsum　rock.　The　variation　in　maximum　radial　free-swelling　ratio,　axial　free-swelling　ratio,　lateral　restricted-swelling　ratio　and　lateral　restricted-swelling　pressure　of　gypsum　rock　with　a　water　immersion　time　of　2880　min　were　investigated　experimentally.　The　early　swelling　characteristics　were　further　discussed　and　described　by　an　S-shaped　model.　The　results　show　that　the　swelling　ratio　and　swelling　pressure　increase　rapidly　as　the　immersion　time　increases　for　the　first　120　min.　Subsequently,　the　swelling　ratio　and　swelling　pressure　increase　slowly　and　become　stable　as　the　immersion　time　further　increases.　At　the　120th　minute,　the　maximum　radial　free-swelling　ratio,　axial　free-swelling　ratio,　lateral　restricted-swelling　ratio　and　lateral　restricted-swelling　pressure　of　gypsum　rock　reach　more　than　80%　of　their　final　values　(2880　min　in　the　present　study).　Based　on　the　swelling　characteristics　of　gypsum　rock　during　the　first　120　min,　an　S-shaped　swelling-time　model　was　introduced　to　describe　the　early　swelling　behavior　of　gypsum　rock.