A Doherty amplifier, comprising: a main amplifier and a peak amplifier; a first microstrip with λ/4 electric length connected between the main amplifier and the peak amplifier; a second microstrip with λ/4 electric length connected between a junction of outputs of the peak amplifier and the main amplifier, and an output terminal; and at least one tuner connected, in series with the first microstrip, between the main amplifier and the peak amplifier, wherein the at least one tuner comprises a third microstrip connected with the first microstrip in series, wherein the at least one tuner is configured to adjust a radius of Voltage Standing Wave Ratio, VSWR, circle of the main amplifier based on at least a characteristic impedance of the third microstrip, wherein the at least one tuner comprises a switch unit for controlling the third microstrip as a shunt, characterised in that, the characteristic impedance and an electric length (θ) of the third microstrip are based on a distance between a load impedance of the main amplifier at a high signal condition and the best efficiency point at a back-off power level, wherein the characteristic impedance of the third microstrip is different from that of the first microstrip, wherein the VSWR radius of the main amplifier equals Zmt*Zload/ZcT 2 where Zmt represents the characteristic impedance of the first microstrip, ZcT represents the characteristic impedance of the second microstrip, and Zload represents a load impedance of the Doherty amplifier, and wherein if the distance is larger than the VSWR radius of the main amplifier, the characteristic impedance of the third microstrip is taken a value lower than that of the first microstrip; and if the distance is smaller than the VSWR radius of the main amplifier, the characteristic impedance of the third microstrip is taken a value larger than that of the first microstrip. a main amplifier and a peak amplifier; a first microstrip with λ/4 electric length connected between the main amplifier and the peak amplifier; a second microstrip with λ/4 electric length connected between a junction of outputs of the peak amplifier and the main amplifier, and an output terminal; and at least one tuner connected, in series with the first microstrip, between the main amplifier and the peak amplifier, wherein the at least one tuner comprises a third microstrip connected with the first microstrip in series, wherein the at least one tuner is configured to adjust a radius of Voltage Standing Wave Ratio, VSWR, circle of the main amplifier based on at least a characteristic impedance of the third microstrip, wherein the at least one tuner comprises a switch unit for controlling the third microstrip as a shunt, characterised in that, the characteristic impedance and an electric length (θ) of the third microstrip are based on a distance between a load impedance of the main amplifier at a high signal condition and the best efficiency point at a back-off power level, wherein the characteristic impedance of the third microstrip is different from that of the first microstrip, wherein the VSWR radius of the main amplifier equals Zmt*Zload/ZcT 2 where Zmt represents the characteristic impedance of the first microstrip, ZcT represents the characteristic impedance of the second microstrip, and Zload represents a load impedance of the Doherty amplifier, and wherein if the distance is larger than the VSWR radius of the main amplifier, the characteristic impedance of the third microstrip is taken a value lower than that of the first microstrip; and if the distance is smaller than the VSWR radius of the main amplifier, the characteristic impedance of the third microstrip is taken a value larger than that of the first microstrip.