Disclosed is a two-storey modular villa building and a construction method thereof. The villa building comprises a foundation, structural upright columns, a gable wall, longitudinal walls, floors and a roof. The structural upright columns are connected with the foundation, the gable wall and the longitudinal walls are connected with the structural upright columns, the floors and the roof are connected with the gable wall and the longitudinal walls, the gable wall comprises a first-storey gable wall and a second-storey gable wall, modular square plate walls are pieced together to form the first-storey gable wall, modular square plate walls and modular triangular plate walls are pieced together to form the second-storey gable wall, each longitudinal wall comprises a first-storey longitudinal wall and a second-storey longitudinal wall, modular square plate walls are pieced together to form the first-storey longitudinal walls and the second-storey longitudinal walls, each modular square plate wall comprises a square external inorganic plate, a square internal inorganic plate and vertical inorganic plate strips parallelly distributed between the square external inorganic plate and the square internal inorganic plate. The modular square plate walls are pieced together to form the first-storey walls and the second storey walls of the villa building, the first-storey walls and the second-storey walls are connected with the floors, so that a firm integral frame is formed and has quite high seismic resistance, and the integral weight of the villa building is greatly reduced.