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In this paper, we calculate three effective refractive indexes for cascaded silicon-on-insulator (SOI) nonlinear directional coupler. For each effective refractive index, the coupling length LC, core-to-distance-ratio K and switching threshold power Pi, m are calculated, respectively. However, there are some fabrication errors, which are ΔW, ΔH, ΔK in the height of Si layer and width of Si layer and error of core-to-distance-ratio. We have analyzed the influence of power transmission efficiency Ti, j and relative transmission efficiency error Δi, j caused by the change of ΔW, ΔH, ΔK. Furthermore, we find the tolerance range of ΔW, ΔH, ΔK for Ti, j and Δi, j in couplers a, b, c. The other four input pulses with different powers are switched to four different ports. Although output pulses show some little compression and loss, the value of η is very small and output pulse waveforms are not affected seriously. The basic switching function has been realized, so this design is proved to be effective.