The　realization　of　high-performance　EMI　shielding　materials　with　flexible,　ultrathin　and　outstanding　shielding　efficiency　is　crucial　in　addressing　the　growing　concern　over　electromagnetic　radiation　from　electronic　devices.　In　this　research,　flexible　and　ultrathin　PDA　modified　Ti3C2Tx　MXene/bacterial　cellulose　(PM/BC)　composite　films　with　densely　stacked　lamellar　structures　have　been　developed　through　a　combination　of　experimental　and　theoretical　investigations.　The　microstructure,　interface　interaction,　mechanical　properties　and　EMI　shielding　efficiency　of　the　composite　films　have　been　comprehensively　investigated.　It　is　highlighted　that　the　strong　interfacial　interaction　between　PDA　and　MXene　nanosheets　contributes　to　the　enhanced　tensile　stress　(178.1　+/-　7.5　MPa)　and　fracture　strain　(6.9　+/-　0.2%)　of　the　flexible　and　ultrathin　PM/BC　composite　film,　demonstrating　its　excellent　resistance　to　external　forces.　More　interestingly,　the　composite　film　exhibits　a　remarkable　electrical　conductivity　of　up　to　39　840　S　m-1　and　a　superior　EMI　shielding　performance　of　53.9　dB,　indicating　its　ability　to　attenuate　electromagnetic　waves　effectively.　These　findings　provide　a　feasible　strategy　to　develop　ultrathin　and　flexible　MXene-based　composite　films　with　exceptional　EMI　shielding　capability,　which　have　great　potential　in　protecting　electronic　devices,　including　wearable　electronic　products.　Ultrathin　and　flexible　PM/BC　films　with　dense　lamellar　structures　have　been　developed　by　experimental　and　theoretical　investigations,　exhibiting　superior　EMI　shielding　performance,　which　show　great　potential　for　the　protection　of　electronic　devices.