A Doherty amplifier (300) is provided, it comprises: a main amplifier (301) and a peak amplifier (302); a first microstrip (303) with λ/4 electric length connected between the main amplifier and the peak amplifier; a second microstrip (304) with λ/4 electric length connected between a junction of outputs of the peak amplifier and the main amplifier, and an output terminal (306); at least a tuner (305) for adjusting radius of VSWR circle of the main amplifier and connected, in series with the first microstrip (303), between the main amplifier (301) and the peak amplifier (302) based on input signal power. The hack-off power level efficiency is increased by enlarge the VSWR radius with the new Doherty structure.