Electromagnetic　nondestructive　evaluation　method　is　feasible　in　assessing　stress　or　residual　stress　levels　in　ferromagnetic　structures,　for　there　is　a　strong　inherent　correlation　between　the　permeability　and　the　stress.　This　study　is　based　on　the　closed　magnetic　circuit　method　for　testing　stress　distribution　along　depth.　Firstly,　a　theory　is　proposed　for　describing　the　propagation　of　electromagnetic　energy　inside　ferromagnetic　materials.　A　layered　model　is　established　considering　the　electromagnetic　field　experienced　different　attenuation　at　different　depths.　And　each　layer　is　assumed　to　have　the　identical　physical　state.　Then,　multi-frequency　eddy　current　method　is　proposed　to　testing　the　in-depth　permeability　variation　and　consequently　the　stress-depth　profile.　For　simply　using　the　convolution　integral,　a　novel　inversion　algorithm　is　given　to　resolve　the　convolution　integral.　To　apply　the　proposed　method　for　testing　the　stress-depth　profile,　multi-frequency　eddy　current　experiments　are　conducted　through　a　four-point　bending　device;　with　the　specimen　material　employed　is　45#　steel.　Finally,　map　the　experimental　data　fitting　curve　to　the　theoretical　calculation　curve,　and　hence　can　verify　the　proposed　theory　and　the　inversion　algorithm.