Flux decline during finite crossflow Ultrafiltration(UF) in charged system was investigated by perceiving membrane fouling as a dynamic process from non-equilibrium to equilibrium. Under the influence of the boundary condition at the initial section and cross flow, the equilibrium is first reached at the initial section of the crossflow filter and the front of the equilibrium region progresses with time toward the end of the filter. A mathematic model was developed to describe this dynamic process of membrane fouling in a charged system. With the model, the time-dependent flux and the time required to reach the steady state in a crossflow filtration can be easily determined. The fouling process under different conditions is simulated as illustrated with a demonstration of the newly developed model. The results of simulation show that there are jumps in the local flux at the front of the equilibrium region and the electrokinetic properties of charged UF system have a very substantial influence on the separation performance of membrane.