With the traffic increase,old road widening is found to be an important issue in the road construction. In order to reduce the different settlement and improve the stability of the embankment,the geosynthetics have been successfully adopted in the joint between the old and new embankments. Based on the features of the geosynthetics- reinforced old road widening,the old and new embankments and the pavement are regarded as a whole structure; and the numerical model is performed on the basis of the assumption. Adopting the FLAC,the mechanism of geosynthetics-reinforced old road widening is discussed mainly from three factors as follows:(1) the displacements of embankment;(2) the stress distributions on the crest of embankment;and (3) the tensions developed in geosynthetics. In addition,the different effects under the different layer numbers of geosynthetics and heights of embankments are studied. The results show that using geosynthetics in the reinforcement of embankment is effective not only in reducing the differential settlements and horizontal displacements on the crest of the embankments,but also in uniform stress distribution that is reflected from embankment to the pavement.