An electronic impedance tuner comprises an adjusting circuit, N cell tuning circuits identical in structure and a switch controller. The adjusting circuit comprises a first microstrip line, a second microstrip line, a first capacitor, a second capacitor, a third capacitor, a first inductor, a second inductor and a first PIN diode. Each cell tuning circuit comprises a third microstrip line, a fourth microstrip line, a fourth capacitor, a fifth capacitor, a second PIN diode and a third capacitor. The capacitance Cd of the fourth capacitor meets the condition: 4 ⁢ ⁢ Y s N ⁢ ⁢ π ⁢ ⁢ f 2 ⁢  Γ req  1 -  Γ req  2 ≤ C d ≤ Y s π ⁢ ⁢ f 1 ⁢  Γ req  1 -  Γ req  2 . The length d of the third microstrip line meets the condition: λ 1 / 4 ⁢ ( N + 1 )