The present disclosure relates to a three-dimensional drainage device suitable for a loose filling slope and methods for constructing the three-dimensional drainage device. The slope includes a stable stratum and a filling soil stratum above the stable stratum. The three-dimensional drainage device includes a surface drainage mechanism, a shallow drainage mechanism and a deep drainage mechanism. The surface drainage mechanism includes one or more catchment canals arranged on an upper surface of the filling soil stratum and a pool arranged on an edge of the filling soil stratum. The shallow drainage mechanism includes one or more first collecting pipes. The deep drainage mechanism includes one or more drainage gabions and one or more second collecting pipes.