Slurry　selection　and　ratio　optimization　are　the　key　points　and　difficulties　in　the　construction　of　subway　tunnels　in　poorly　graded　medium　sand　strata.　In　this　paper,　a　kind　of　acidic　grouting　material　was　prepared　by　using　sodium　silicate　and　phosphoric　acid.　The　properties　of　the　acidic　grouting　material　including　gel　time　and　viscosity　were　studied.　Orthogonal　test　was　carried　out　to　optimize　the　ratio　of　sodium　silicate　to　phosphoric　acid　for　poorly　graded　sand　stratum.　At　the　same　time,　X-ray　diffraction　(XRD)　and　scanning　electron　microscope　(SEM)　were　used　to　analyze　the　sand　consolidation　process　of　the　slurry.　The　results　show　that　the　gel　time　and　funnel　viscosity　of　the　phosphoric　acid-sodium　silicate　slurry　are　related　to　the　Baume　degree　of　sodium　silicate　and　the　volume　ratio　of　sodium　silicate　to　phosphoric　acid.　The　compressive　strength　of　the　grouted　sand　is　related　to　four　factors,　i.e.,　water　content　of　the　sand,　and　Baume　degree,　pH　value　and　curing　time　of　the　slurry,　the　in-fluence　degree　of　which　is　in　the　following　order:　D　(curing　time)　＞　A　(water　content)　＞　B　(Baume　degree　of　sodium　silicate)　＞　C　(pH　value　of　the　slurry).　The　final　slurry　selected　for　grouting　reinforcement　of　the　weakly　alkaline　medium　sand　strata　was　prepared　by　30　degrees　Be　&　PRIME;　sodium　silicate　with　14　%　phosphoric　acid　at　a　volume　ratio　of　0.5.　It　has　a　gel　time　of　51.48　min,　a　funnel　viscosity　of　16.43　s　and　a　pH　value　of　3.47.　The　increased　amount　of　silica　gel　in　the　sample　is　the　main　reason　for　the　increased　strength　of　the　grouted　sand.　The　amount　of　silica　gel　generated　increases　with　the　increase　of　the　curing　time　of　the　sample.