Based on the second-order nonlinearity, we present a bidirectional tunable all-optical switch at C-band by introducing backward quasi-phase-matching technique in Mg-doped periodically poled lithium niobate (MgO:PPLN) waveguide with a nano-structure called multiple resonators. Two injecting forward lights and one backward propagating light interact with difference frequency generations. The transmission of forward signal and backward idler light can be modulated simultaneously with the variation of control light power based on the basic “phase shift” structure of a single resonator. In this scheme, all the results come from our simulation. The speed of this bidirectional optical switch can reach to femtosecond if a femtosecond laser is used as the control light.