A typical electrode reaction of electro-organic synthesis usually takes place with competing side electrode reaction and homogeneous reaction.To enhance the electrode polarization for increasing reaction rate will considerably reduce the reaction selectivity.Packed bed electrodes can reach higher current densities at relatively low electrode polarizations，which is favorable to mitigating the inconsistency between reaction rate and selectivity.In this paper，the typical process of electro-organic synthesis in a differential reactor of packed bed electrode（PBEDR），was theoretically analyzed with emphasis on the effect of lateral distribution of over-potential on the selectivity.A generalized mathematical model was developed to describe the distribution of over-potential.Dimensionless variables μ and ω that characterize the polarization and influence of side electrode reaction in the system，were derived from modeling.Adomian’s decomposition method(ADM）was used to solve the nonlinear differential equation of the model to obtain an approximate analytical solution，which was in the form of algebraic expressions of infinite power series.By the solution，the relationship between over-potential distribution and average selectivity could be easily calculated without solving the nonlinear model time and again.Finally，an analysis of electrochemical reduction of nitrobenzene in a PBEDR was presented，and the characteristic size of the reactor，namely the thickness of the packed bed electrode，was theoretically optimized.The results showed that the calculated results were consistent with experimental data satisfactorily.