Although　organic　electrode　materials　have　merits　of　abundant　resources,　diverse　structures　and　environmental　friendliness,　their　performance　for　electrochemical　energy　storage　is　far　insufficient.　In　this　work,　a　thiourea-based　polyimide/reduced　graphene　oxide　(PNTCSA/RGO)　composite　was　synthesized　via　a　condensation　polymerization　method.　As　a　cathode　material　in　lithium-ion　batteries,　excellent　performance　is　demonstrated　with　high　reversible　capacity　(144.2　mA　h　g−1),　high　discharge　voltage　(∼2.5　V),　and　long　cycling　life　(over　2000　cycles　at　500　mA　g−1),　which　are　comparable　to　those　of　other　well　documented　in　organic　electrodes.　Encouraging　electrochemical　performance　is　also　demonstrated　for　sodium　ion　batteries　(a　cycling　life　of　800　cycles　at　500　mA　g−1),　while　poor　performance　is　delivered　in　potassium　ion　batteries.　Theoretical　studies　reveal　that　the　active　sites　are　carbonyl　groups　for　all　alkali　ions　but　one　inserted　alkali　metal　ion　is　shared　by　two　carbonyl　groups　from　the　two　neighbor　units.　More　importantly,　K　ions　have　stronger　interaction　with　S　atoms　than　Li/Na　ions,　which　may　lead　to　poor　structure　reversibility　and　account　for　the　poor　cycling　performance.　Our　findings　provide　a　fundamental　understanding　of　polyimide　based　polymer　electrodes　and　help　to　design　and　develop　high　performance　organic　electrode　materials　for　alkali　metal　ion　batteries.　©　2020,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.