In　this　paper,　a　new　method　is　proposed　from　the　FDTD　model　to　solve　the　challenge　that　the　digital　signal　source　is　time-harmonic　or　single　frequency　reconstruction.　The　new　method　does　not　need　to　know　Green＇s　function　and　identify　the　peak　value.　Through　in-depth　exploration　and　improvement　of　the　frequency　domain　time-reversal　method,　the　source　condition　of　single　frequency　sources　is　derived.　A　numerical　example　is　given　to　verify　the　effectiveness　of　the　proposed　method.　This　work　provides　another　way　to　further　develop　practical　electromagnetic　source　reconstruction　techniques　and　test　systems.