In this study, we conducted nearly 100 crystallization experiments using granitic glass with varying initial boron contents under conditions of 650–800 °C, 300 MPa, aH2O = 0.5~1 and fO2 conditions of ~NNO and NNO+2.3. These experiments aimed to determine the conditions required for tourmaline crystallization in granitic melts. The results highlight that boron content is the primary factor governing tourmaline saturation in granitic melts. Boron concentration required for tourmaline crystallization increases significantly with temperature, ranging from ~0.5 wt% at 650 °C to > 2 wt% at 750 °C. Furthermore, a decrease in water activity and an increase in oxygen fugacity promote tourmaline crystallization. We have constructed phase diagrams plotting B2O3 concentration in the melt against temperature, delineating the tourmaline saturation boundaries under varying water activity and oxygen fugacity. In contrast to previous studies, no obvious correlation between the aluminum saturation index (ASI) and tourmaline crystallization was observed in our experiments. Tourmaline was found to be stable even in metaluminous melts (ASI < 1.1) provided the melts contained sufficient boron. The mineral assemblages observed in the experimental runs indicate that the liquidus phase of boron-bearing granitic melts is controlled by both boron concentration and water activity. Tourmaline can serve as the liquidus phase in granitic melts with high boron content and high water activity (aH2O > 0.7). In some experiments, another boron-rich phase, dumortierite, also crystallized and acted as the liquidus phase under conditions of high boron content and low water activity. In contrast, biotite was the liquidus phase in melts with low boron content and high water activity, while plagioclase crystallized first in melts with low boron and low water activity.