  Recently, it has been pointed out that the twisting of bilayer WSe$_2$ would
generate topologically non-trivial flat bands near the Fermi energy. In this
work, we show that twisted bilayer WSe$_2$ (tWSe$_2$) with uniaxial strain
exhibits a large nonlinear Hall (NLH) response due to the non-trivial Berry
curvatures of the flat bands. Moreover, the NLH effect is greatly enhanced near
the topological phase transition point which can be tuned by a vertical
displacement field. Importantly, the nonlinear Hall signal changes sign across
the topological phase transition point and provides a way to identify the
topological phase transition and probe the topological properties of the flat
bands. The strong enhancement and high tunability of the NLH effect near the
topological phase transition point renders tWSe$_2$ and related moire materials
new platforms for rectification and second harmonic generations.
