This paper shows an asymptotic expansion of Green's function for a two-layered acoustic halfspace. In general, an asymptotic expansion is able to give a brief overview for the physical phenomena at the far field from a source point by indicating the decaying factor, although, there is a difficulty to represent the asymptotic form due to the complicated modification of the wavenumber integral. In this paper, a proper modification of the wavenumber integral path is carried out to apply Watson's lemma, namely the wavenumber integration is transformed into Laplace transform. The asymptotic expansion obtained here is applicable to a generalized layered medium because of incorporating the representation of Green's function in terms of eigenvalues and takes some properties that are not found out through Green's function directly.