The invention discloses an indoor wall body plate installing system and an installing method thereof. A plurality of wall body plates and an installing assembly are provided, wherein the installing assembly is an installing system compounded by leveling plate strips, a straight wall installing unit, an opening-closing strip, connecting units, an external corner installing unit, an internal corner installing unit, an installing fastener and a skirting line. The installing assembly is installed by matching with the wall body plates, a keel is not needed, space is saved, the disassembly and the assembly are convenient and fast, the construction efficiency is improved, and the installing cost is lowered; the installing assembly and a wall body plate assembly can be standardizationally produced, the flatness is high after installation, and the overall effect is good; the wall body plates are not in direct contact with wall bodies, gaps are reserved between the wall body plates and the connecting units, and thus the deformation caused by the change of indoor environment is avoided, and the service life of the wall body plates is prolonged.