The invention relates to an installation method of an indoor suspended ceiling and a partition wall decorative board, belonging to the indoor decoration suspended ceiling technical field. The installation method comprises the fixed installation of an indoor suspended ceiling and a partition wall decorative board as well as the joint filling and seaming treatment of the decorative board. The fixed installation of the decorative board comprises the following steps: firstly, the decorative board is fixed on a wall body and a ceiling by setscrews; an inner layer support is welded on the setscrews by a round reinforcing steel bar with the diameter equal to 12 mm, thereby fixing the inner layer support as a main rib; the main rib of the inner support of a fixed angle steel is fixed as the auxiliary rib of an outer support by a light steel keel; a decorative board which is nailed on the auxiliary rib of the blue steel keel through bolts is fixed with the light steel keel; the outer support of the light steel keel is connected with the wall body, the partition wall and the ceiling through setscrews; the decorative board and circumferential joint heads are respectively fixed with the light steel keel through bolts. The joint filling and seaming treatment of the decorative board comprises the following step: split-preventing and joint-filling powder is adopted for joint filling and seaming and three-layer glass fiber adhesive tape is adopted as a framework. Moreover, the installation method can reach overall straight and smooth.