The invention discloses a combined window casing and an installation method thereof. The window casing comprises side table plates, a top table plate, a lower table plate, side attaching panels, a topattaching panel, a lower attaching panel and a base plate, wherein the top table plate is arranged between the side table plates, the top attaching panel is arranged between the side attaching panels, the side attaching panels are arranged on the two sides of the lower attaching panel, the side table plates are symmetrically distributed on the lower table plate, and the base plate is located between the lower table plate and the side table plates. The installation method comprises the following steps of (a) window opening base surface treatment; (b) supporting and positioning; (c) gluing andfixing; (d) edge line installation; and (e) installing and forming. The window casing is simple in structure, convenient to transport, rapid and convenient during field construction, flexible to assemble and easy to adjust, and the surface does not expose nail holes and is flat and uniform; the attractiveness is high, the later maintenance is facilitated, the installation method is reasonable andorderly, saves labor and is convenient, construction intensity is low, the operation is simple, the construction space is large, the control and the adjustment are easy, and the installation accuracyand the overall attractiveness can be improved.