The invention discloses a dual-mode power amplifier with switchable working power and a mode switch method. The power amplifier adopts a multi-tap input transformer, and realizes the switching between preload line and output load line by controlling the on/off of the intermediate switch connected with taps, so as to achieve the best power conversion efficiency under different maximum output powers. By using the change-over switch to control the capacitance value of the matching capacitor, it is easier to adjust the load line, thus further ensuring the performance of the power amplifier provided by the invention. The intermediate switch and change-over switch are integrated on an independent chip by CMOS/phemt/bihemt/SeGe/SOI,etc, or on a power amplifier chip by CMOS/phemt/bihemt/SeGe/SOI, etc, which is easy to realize.