A novel tunable phase shifter (PS) with large phase range is proposed based on the reconfigurable resonator. The PS composes of one switchable resonator with compact size and two external coupling networks (ECNs). The resonator is designed to switch between two states with a large constant phase difference and with different matching impedances. The ECNs are used to match the changeable impedance and meanwhile shift the phase continuously. The switchable resonator cooperates with the ECNs to complete the entire PS design, doubling the phase shift range (PSR) to 180°. The systematic design method taking phase performance into consideration is put forward. The designed PS exhibits good performance with 10-dB fractional bandwidth of 13.3%, a large PSR of 0° - 180°, and a low phase error of ± 3.5°.