The　eddy-current　testing　technique　has　been　extensively　explored　for　the　measurement　of　metallic　pipes　in　various　industrial　processes.　In　this　study,　a　simplified　analytical　model　of　metallic　pipes　encircled　by　a　coaxial　sensor　is　derived　according　to　the　asymptotic　forms　of　modified　Bessel　functions.　The　original　Dodd　and　Deeds　(DD)　model　is　thus　approximated　by　an　elementary　function　describing　the　relationship　between　the　measured　inductance　spectrum　and　the　pipe　properties.　Due　to　the　simple　nature　of　the　elementary　function,　the　Jacobian　matrix　can　be　evaluated　much　more　efficiently.　Accordingly,　a　fast　Newton-Raphson　method-based　optimization　approach　is　developed.　The　proposed　method　has　been　evaluated　by　comparing　it　with　the　numerical　simulation　of　the　finite-element　method　(FEM)　and　experimental　measurements　which　indicate　its　effectiveness　for　the　measurement　of　metallic　pipe　properties.