Organic　semiconductors　(OSCs)　are　featured　by　weak　spin-orbit　coupling　due　to　their　light　chemical　element　composition,　which　enables　them　to　maintain　spin　orientation　for　a　long　spin　lifetime　and　show　significant　potential　in　room-temperature　spin　transport.　Carrier　mobility　and　spin　lifetime　are　the　two　main　factors　of　the　spin　transport　performance　of　OSCs,　however,　their　ambiguous　mechanisms　with　molecular　structure　make　the　development　of　spintronic　materials　really　stagnant.　Herein,　the　effects　of　halogen　substitution　in　bay-annulated　indigo-based　polymers　on　carrier　mobility　and　spin　relaxation　have　been　systematically　investigated.　The　enhanced　carrier　mobility　with　an　undiminished　spin　lifetime　contributes　to　a　3.7-fold　increase　in　spin　diffusion　length　and　a　record-high　magnetoresistance　of　8.7%　at　room　temperature.　By　analyzing　the　spin-orbit　coupling　and　hyperfine　interaction,　it　was　found　that　the　distance　of　the　substitution　site　from　the　conjugated　center　and　the　nitrogen　atoms　in　the　molecules　play　crucial　roles　in　spin　relaxation.　Based　on　the　above　results,　we　proposed　a　molecular　design　strategy　of　halogen　substitution　far　from　conjugate　center　to　enhance　spin　transport　efficiency,　presenting　a　promising　avenue　for　advancing　the　field　of　organic　spintronics.