We designed a novel and low switching-power all-optical fiber switch. The highly nonlinear photonic crystal fiber (PCF) and the bidirectionally pumped Er3+ doped fiber amplifier are inserted into the Sagnac loop mirror simultaneously. Therefore, the symmetry of the loop is broken, and the switching function is realized for the phase shift of the reversely propagating signal. The theoretical analysis shows that the switching power is inversely proportional to the product of the amplifier gain and the PCF nonlinear coefficient. In the experiment, 40 mW switching power and 15.9 dB switching extinction are obtained, furthermore, the transmission of signal light is cosine proportional to the peak power of the pump pulse. The experimental result agrees well with that from theory.