The invention discloses a lightweight high-strength calcium silicate board and a preparation process thereof. The light high-strength calcium silicate board is composed of the following components, inpercentages by mass: 10-20% of cement, 35-60% of quartz sand powder, 10-20% of slaked lime powder, 5-10% of fly ash, 5-10% of reinforced cellulose, 2-5% of a filler, 5-10% of bentonite, and 1-3% of aflocculating agent. The preparation process includes the steps of fiber slurry preparation, quartz sand slurry preparation, pulping, defibrination and pre-stirring, board preparation, pre-curing, autoclaved curing, drying, and shaping. According to the lightweight high-strength calcium silicate board provided by the invention, a synergistic effect of different materials is fully played through ingenious proportioning of the materials, especially the cement, the slaked lime powder, the fly ash and the bentonite are adopted, so that the difficult problem that mortar or concrete is easily cracked because quicklime and cement are directly adopted in a traditional method can be avoided, formation of tobermorite crystals in the preparation process can be promoted, and the strength and qualification rate of the calcium silicate board can be improved.