In order to decrease the volume and increase the practicality， it is an attractive scheme of realizing the Raman laser by using the modulation of an electro-optic modulator. However， this way produces additional laser lines of a Raman laser， which gives a big impact on absolute gravity measurement. In order to ensure the accuracy of measuring the absolute gravity， we have realized the optimization and evaluation of this maximum system error in this paper， combined by the theoretical model and the modulation experiments. The result has proved the evaluation method and the uncertainty of this effect is evaluated to 20 μGal.