The SNR caused by modulation instability (MI) in transmitter fibers is given. As input signal power P reaches a larger value, MI may degrade SNR. Corresponding to the maximum SNR, there are a suitable P and a transmitter length /. The modulation sidebands △λc is also studied. △λc increases fastly with P. To limit the MI effect to P, channel spacing AA must be bigger than △λc, but bigger P(>20 dBm) will caused the MI effect and make SNR worse.