Functional refractory materials for flow control devices of molten steel in continuous casting used to be prepared from Al2O3-C refractories containing dense corundum aggregates. According to the traditional concept, the denser the refractories, the higher the strength of refractories. However, we prepared a new lightweight Al2O3-C refractory material using microporous corundum aggregates instead of dense corundum aggregates, which were reinforced by in situ formed SiC whiskers. A comparative analysis of microstructures and properties was carried out for conventional and lightweight Al2O3-C refractories with and without Si powder addition. We showed that microporous aggregates formed a better aggregate/matrix interface bonding and an improved distribution of SiC whiskers. The SiC whiskers formed not only in the matrix, but also inside of the microporous aggregates and at the aggregate/matrix interface by a vapor-solid reaction mechanism. Due to the formation of a microporous aggregate/matrix interface reinforced by SiC whiskers, the crack propagation along the aggregate/matrix interface was suppressed, whereas the percentage of cracks propagating within the aggregates was enhanced. Thus, the synergy between in situ formed SiC whiskers and microporous aggregates resulted in a significant higher strength of lightweight Al2O3-C refractories compared to conventional ones. The results therefore provide an original strategy to strengthen Al2O3-C refractories.