Polydimethylsiloxane　(PDMS)　has　been　widely　used　in　gas　separation,　pervaporation,　and　organic　solvent　nanofiltration.　The　crosslinking　and　penetration　behavior　of　PDMS　has　been　of　great　importance　for　fabricating　composite　membranes.　In　this　study,　an　in-situ　low-field　nuclear　magnetic　resonance　(LF-NMR)　spectroscopy　was　employed　to　probe　the　crosslink　density　of　PDMS　during　the　crosslinking　process.　The　relationship　between　crosslink　density　and　nanostructure　of　PDMS　selective　layer　was　systematically　investigated.　It　was　found　when　the　PDMS　network　had　low　crosslink　density　(upsilon　e　＜　1.32　x　10-3　mol　cm-3),　the　un-crosslinked　PDMS　chains　could　easily　infiltrate　into　the　pores　of　the　substrate,　leading　to　a　defective　active　layer.　Therefore,　the　rejection　for　Evans　blue　from　organic　solvents　was　below　3.9%.　With　the　increasing　crosslink　density　to　1.57　x　10-3　mol　cm-3,　the　integrity　of　the　PDMS　selective　layer　was　remarkably　reinforced　with　a　rejection　of　99.0%.　Moreover,　the　asprepared　membrane　exhibited　good　performance　for　concentrating　trace　vitamin　B12　from　different　solvents.　The　swelling　property　was　responsive　monitored　with　T2　relaxation　time　using　LF-NMR.