  In this article, we compute the limit of the Wang--Yau quasi-local mass on a
family of surfaces approaching the apparent horizon (the near horizon limit).
Such limit is first considered in [1]. Recently, Pook-Kolb, Zhao, Andersson,
Krishnan, and Yau investigated the near horizon limit of the Wang--Yau
quasi-local mass in binary black hole mergers in [12] and conjectured that the
optimal embeddings approach the isometric embedding of the horizon into $\R^3$.
Moreover, the quasi-local mass converges to the total mean curvature of the
image. The vanishing of the norm of the mean curvature vector implies special
properties for the Wang--Yau quasi-local energy and the optimal embedding
equation. We utilize these features to prove the existence and uniqueness of
the optimal embedding and investigate the minimization of the Wang--Yau
quasi-local energy. In particular, we prove the continuity of the quasi-local
mass in the near horizon limit.
