The static yield stress of Electrorheological Fluid(ERF) was calculated. The result is in good accordance with the experiment. Using a particle chain model to estimate the interactions of particles, we solved Maxwell equation by the finite differential method, and calculated Maxwell stress directly. In this method, both multipole and manybody effects between particles, which were not included or were treated inappropriately before, were treated exactly. The relationship of the static yield stress to the electric field strength and the particle size is also derived from our model.