The invention discloses a magnesium-aluminum-zirconium composite brick with low thermal conductivity. The magnesium-aluminum-zirconium composite brick comprises the following raw materials by weight: 10 to 65% of microporous magnesia, 25 to 65% of high-purity magnesia, 5 to 20% of fused magnesium aluminate spinel and 1 to 10% of zirconium dioxide. The composite brick further comprises waste paper pulp accounting for 2 to 5% of the total weight of the above raw materials. The microporous magnesia is prepared by subjecting a microporous magnesia synthetic material to fragmentation and screening. The magnesium-aluminum-zirconium composite brick with low thermal conductivity in the invention has the advantages of high apparent porosity, low volume density, low thermal conductivity, high-temperature resistance, erosion resistance, scouring resistance and integrated fireproof and heat-insulating functions, is applicable to a high-temperature zone of a rotary kiln, can substantially reduce heat loss in a kiln body and produces good energy-saving and emission-reducing effects.