The　performance　monitoring　of　fiber-optics　communication　is　an　important　task　in　nowadays　communication　system.　Link　optical　noise-to-signal　ratio　(OSNR)　is　one　of　the　most　important　parameters　that　affect　the　performance　of　optical　networks.　The　traditional　internal　measurement　method　may　increase　the　network　construction　cost　and　operation　complexity.　To　overcome　these　drawbacks,　an　ANN　based　link　OSNR　estimation　method　with　external　measurement　is　proposed　in　this　paper.　Route　level　OSNR　values　are　measured　at　the　edge　nodes　and　are　used　for　link　level　OSNR　estimation　with　the　trained　ANN.　Besides,　a　heuristic　method　for　route　set　generation　is　proposed　to　generate　the　route　set　that　introduce　fewer　extra　network　load.　The　experiment　results　demonstrate　that　the　ANN　based　method　can　meet　the　practical　requirement　in　both　estimation　accuracy　and　computation　complexity.　The　proposed　method　can　be　an　important　part　of　optical　network　OSNR　monitoring　to　ensure　robust　and　intelligent　network　operation.