In　order　to　improve　the　efficiency　of　Digital　TV　transmitter,　we　uses　a　new　type　of　four-stage　Doherty　structure.　Based　on　the　transmission　line　theory　and　active　loadpull　theory,　we　deduce　the　principle　of　the　four-stage　Doherty　structure,　and　use　ADS　to　design　a　four-stage　Doherty　power　amplifier.　The　simulation　result　shows　that　this　four-stage　Doherty　PA　has　an　efficiency　of　34%　at　6dB　back-off　(53dBm)　for　DVB-T　OFDM　64　QAM　modulated　signal.　The　ACPR　and　MER　obtained　at　53dBm　when　output　power　are　31dBc　and　28dB　respectively.