van der Waals layered α-In2Se3 has shown out-of-plane ferroelectricity down to the bilayer and monolayer thicknesses at room temperature that can be switched by an applied electric field. This work addresses the missing theoretical framework through a comprehensive study on the layer-dependent electronic structure, ferroelectricity and the inter-layer interaction of α-In2Se3, by using first-principles density functional theory. Furthermore, surface states and their response to the built-in internal depolarizing field were carefully analyzed. Phase transition and Curie temperatures of 1L α-In2Se3 were studied by employing Monte Carlo and ab initio molecular dynamics simulations. The estimated Curie point is above room temperature, making 1L α-In2Se3 a promising candidate for future ultra-thin ferroelectric devices.