The annihilation cross section of the dark matter is one of the most important quantity for the phenomenology of the dark matter. If the dark matter couples with a light force carrier, non-perturbative resummation is required in non-relativistic regime due to so-called Sommerfeld enhancement. A commonly used formula of this cross section is a product of the hard (short-range) scattering cross section and the enhancement factor which is calculated by dark matter two-body wave function. However, it is known that this formula violates the unitarity bound for small velocity if there is zero energy bound state. In this talk, I will discuss the calculation of the dark matter annihilation cross section in non-relativistic quantum mechanics. In our framework, both short-range and long-range effect is treated in a non-perturbative way, and the cross section satisfies the unitarity bound.