With the increasing demands for the capacity of fiber communication system nowadays，DWDM and L-band optical devices had become research hot spots．Moreover, multi-wavelength source has attracted more and more attention. The theory model of electro-optic intensity modulation based on the structure of Mach-Zehnder is illustrated. The output optical spectrum of broadening result is simulated and analyzed by the formulae while the microwave frequency and power are fixed. The 10 GHz, 15 GHz and 20 GHz frequency shift at 1 550 nm are obtained experimentally with a LiNbO3 waveguide. The spectrum calculated by theory agrees with the result of experimental system in real shape, and optical sidebands with equal frequency intervals are observed in the output spectrum in simulation. The third-order sidebands can be observed distinct when the frequency is 10 GHz, the modulation depth is 1.39 and the direct current bias voltage is 2.98 V. Particularly, the fundamental frequency and the first sideband obtain the same peak power means which can generate a three-wavelength optical source of excellence.