Recently,　Boyer　et　al.　(Phys　Rev　Lett　99:140501,　2007)　introduced　a　conceptually　novel　semi-quantum　key　distribution　scheme　(BKM07).　Tan　et　al.　(Phys　Rev　Lett　102:098901,　2009)　showed　that　classical　Bob　is　unable　to　detect　Eve＇s　eavesdropping　by　giving　a　special　implementation　of　BKM07　protocol.　In　the　reply,　Boyer　et　al.　(Phys　Rev　Lett　102:098902,　2009)　gave　a　solution　against　the　eavesdropping,　i.e.,　Bob　may　place　a　filter　that　allows　only　photons　with　approximately　specific　frequency　to　pass　just　at　the　expected　time　.　However,　their　improvement　contradicts　the　descriptions　about　＂classical.＂　If　the　assumption　of　＂classical＂　is　not　considered,　we　give　a　delay-photon　Trojan-horse　attack　on　BKM07　protocol　and　its　improvement　and　further　present　a　possible　improvement.