For application of high-temperature structural materials, in-situ Si3N4 reinforced BAS composites have been explored. The main drawback for thermal cycling use is the persistent being of hexacelsian BAS phase, which will cause a volume change around 300°C. The effective additives, like Li2O can promote the polymorphic conversion of hexacelsian to celsian. The XRD experiments show that in monolithic system the Li2O-containing celsian can persistent exist at 1650°C; however, in BAS/Si3N4 composite it transforms into hexacelsian pahse at 1600°C. It is found that the presence of Si3N4 could enhance the instability of the Li2O-containing celsian above 1590°C, in which the hexacelsian is a stable phase in thermodynamics. The instability mechanism is indirectly verified as the doping effect of Si, which is coming from thermal decomposition of Si3N4 in the composites. It is well known that Si or SiO2 is a network former, which could make crankshaft-like chain structure of celsian instable and enhance the layer structure of hexacelsian formed.