Halo distribution is a key topic for background study. This paper has developed an analytical method to give an estimation of beam halo distribution in storage rings. This is a creative new theory. As an example, the equilibrium particle distribution of the beam tail in the Accelerator Test Facility damping ring is calculated analytically with different emittance and different vacuum degree. The analytical results agree the measurements very well. This is a general method, which can be applied to any electron rings.