A　geometric　Monte-Carlo　(GMC)　algorithm　is　proposed　based　on　accurate　calculation　of　optical　transmission　on　the　material　interface　for　the　simulation　of　light　propagation　in　turbid　media　(e.g.　bio-tissue).　Grid　is　not　necessary　for　the　computation　of　photons＇　motion　and　the　memory　of　optical　parameters　can　be　saved.　Optical　transmission　is　exactly　simulated　at　material　interfaces　with　curved　boundary　according　to　the　principles　of　geometric　optics,　while　energy　deposition　in　different　structures　can　be　precisely　counted.　The　validity　of　GMC　method　is　confirmed　by　the　comparison　with　voxels　based　Monte-Carlo　method.　Time　consumed　by　GMC　method　is　dramatically　shortened　compared　with　the　hexahedral　voxels　based　Monte-Carlo　(VMC)　method:　about　25　times　faster　than　VMC　with　a　grid　of　140×　140×　100　cells.　In　addition,　the　present　GMC　method　shows　better　portability　when　raising　the　geometric　complexity.