According to the results of experiments and theoretical analysis, a phenomenon called "capture effect" is put forward, which could be used to describe the particles dynamic behavior of electrorheological (ER) suspensions. Then a "structure-force" mathematical model is established to explain this effect based on electrostatic energy density equation. The analysis results show that the dynamic coupling process of ER suspensions under an external electric filed is the function not only of the electric intensity, but also of the dielectric properties and the structure form.