The bandwidth of the coplanar traveling-wave electroabsorption modulator(TW-EAM) was analyzed in the aspect of designing electrode. The calculation process of this method was introduced by demostrating designing a coplanar TW-EAM with its measured data. The demostrating result tallies with the result of experimental measure. It was analyzed the effect of the width of middle electrode and the electrode interval on the bandwidth. An example is also introduced that a GaAs/GaAlAs multiple quantum wells(MQWs) TW-EAM with alarge bandwidth which was designed by using this method.