Based on improved TOAD and introducing DC assistant light which solves two groups signals to compete in SOA and realizes transformations from three condition to two condition,a novel scheme to implement header recognition for NRZ was proposed.Simulation results shows that this program can get 13.29 dB extinction ratio(ER) of the output at 40Gb/s with common SOA.The analysis of impact of nonlinear phase shift difference departure from π and 2π in actual system shows that when a fluctuant rang of nonlinear phase shift difference is confined within 0.1 rad,and 13 dB extinction ratio of recognition signal is still obtained with assistant light injection.Therefor the demander of debugging is degraded greatly.Its validity is confirmed through experiments at 2.5 Gb/s.The experimental result is agree with the theoretic analysis.Further more,the influencing factors to the ER of recognition signal was researched in experimentatlly.