A low switchingpower alloptical switch was designed. The erbiumdoped fiber and the microstructure fiber were inserted into the Sagnac loop mirror simultaneously. Under the effect of the pump modulation, signal light was amplified when passing the erbiumdoped fiber. The symmetry of the loop was broken, two counterpropagating signal lights in the loop acquire nonlinear phase due to the crossphase modulation effect, and the light switch was realized. The theoretical analysis shows that while the signal light goes through erbiumdoped, the larger gain it gets, and the higher coefficients of the microstructure fiber is, the lower switch power is. Furthermore, the transmission rate of signal light in the loop varies with the phase difference of the two counterpropagating signal lights into cosine changes. the simulation results shows that switching power is 26.73 mW, which agrees with the theory.