The invention, belonging to the field of building materials, particularly relates to a lightweight thermal insulation sheet material for building walls. The thermal insulation sheet material comprises the following ingredients: 8.5-10.5 weight portions of fly ash floating bead, 30-38 weight portions of closed cell perlite, 43-52 weight portions of cement, 7-8.5 weight portions of bentonite, 0.04-0.06 weight portions of cellulose ether, 0.07-0.09 weight portions of short fiber, 0.04-0.06 weight portions of wood fiber, and 0.3-0.4 weight portions of gelatine powder. The thermal insulation sheetmaterial is prepared by using organic materials, and has the characteristics of no toxicity, good thermal insulation property, high temperature resistance, low temperature resistance, non-inflammableproperty, high efficiency, and aging resistance. When the thermal insulation sheet material is used for building walls, the thermal insulation property of the sheet material can not be reduced with time, and the sheet material has the same service life with the building.