The　high-capacity　phosphorus-　(P-)　based　anode　materials　for　sodium-ion　batteries　(NIBs)　often　face　poor　performance　retentions　owing　to　the　low　conductivity　and　large　volume　expansion.　It　is　thus　essential　to　buffer　these　problems　by　appropriately　alloying　with　other　elements　such　as　tin　(Sn)　and　constructing　well-designed　microstructures.　Herein,　a　series　of　P-/Sn-based　composites　have　been　synthesized　by　the　facile　and　low-cost　one-step　ball　milling.　Pair　distribution　function　(PDF)　has　been　employed　as　a　hardcore　quantitative　technique　to　elucidate　their　structures　combined　with　other　techniques,　suggesting　the　formation　and　ratios　of　Sn4P3　and　Sn　crystalline　domains　embedded　inside　an　amorphous　P/carbon　matrix.　The　composite　with　the　largest　amount　of　Sn4P3　in　the　P/C　matrix　can　deliver　the　most　balanced　electrochemical　performance,　with　a　capacity　of　422.3　mA-h　g(-1)　for　300　cycles　at　a　current　density　of　1000mAg(-1).　The　reaction　mechanism　has　been　elucidated　by　Na-23　and　P-31　solid-state　nuclear　magnetic　resonance　(NMR)　investigations.　The　study　sheds　light　on　the　rational　design　and　concrete　identification　of　P-/Sn-based　amorphous-dominant　composite　materials　for　NIBs.