  In this paper, we proved a local arithmetic Siegel-Weil formula for a $U(1,
1)$-Shimura variety at a ramified prime, a.k.a. a Kudla-Rapoport conjecture at
a ramified case. The formula needs to be modified from the original
Kudla-Rapoport conjecture. In the process, we also gives an explicit
decomposition of the special divisors of the Rapoport-Zink space of unitary
type $(1, 1)$ (Kr\"amer model). A key ingredient is to relate the Rapoport-Zink
space to the Drinfeld upper plane.
