The　time-reversal　(TR)　method　has　been　proven　effective　in　reconstructing　sources　due　to　its　spatiotemporal　focusing　properties.　Much　work　so　far　on　the　topic　has　been　conducted　with　sources　of　specific　frequency　bandwidths.　However,　most　sources　in　practical　situations　are　time-harmonic　or　single　frequency.　This　letter　proposes　a　novel　approach　to　address　this　challenge　by　analyzing　the　TR　method　in　the　frequency　domain　and　modifying　it　to　derive　a　source　condition　for　single-frequency　sources.　The　proposed　method　is　validated　through　numerical　examples,　demonstrating　its　effectiveness　in　source　reconstruction.　This　work　provides　an　alternative　pathway　for　further　development　of　practical　electromagnetic　source　reconstruction　techniques　and　testing　systems.