Titanium dioxide nanoparticles (TiO2 NPs) can cause mitochondrial apoptosis of TM4 cells, but the mechanisms behind this process are not yet clear. The aim of this study was to evaluate if the accumulation of ROS caused by TiO2 NPs inhibits MCUb expression, causing mitochondrial calcium overload and ultimately leading to cell apoptosis through the mitochondrial pathway. TM4 cells were exposed to different concentrations of TiO2 NPs (0, 25, 50, 75, 100 µg/mL) for 24 hours. We measured the cell viability, ROS level, MCUb and VDAC1 expression, mitochondrial and cytoplasmic Ca2+ level, MMP, apoptosis rate, and the key proteins related to apoptosis via the mitochondria pathway (Bcl-2, Bax, Caspase 3, Caspase 9, p53 and Cyt c). The effect of NAC on MCUb expression, calcium homeostasis, and cell apoptosis were also measured in this study. The results showed that compared to TM4 cells in control group, TiO2 NPs significantly increased ROS level, downregulated MCUb expression, prompted the Ca2+ level in mitochondria and cytoplasm, and enhanced the mitochondria-regulated apoptosis, starting from the 50 µg/mL TiO2 NPs group. However, NAC significantly increased the expression of MCUb, attenuated Ca2+ level in mitochondria and cytoplasm, and reduced the mitochondria-related apoptosis of TM4 cells compared with those in TiO2 NPs group cells. In conclusion, TiO2 NPs induced ROS accumulation which inhibits the expression of MCUb. The deceased MCUb level leads to Ca2+ overload in mitochondria, which causes TM4 cells apoptosis through the mitochondrial pathway. The results of this research elucidate the role of ROS in regulating mitochondrial calcium overload through MCUb for the first time when TM4 cells were exposed to TiO2 NPs, and the results also supplement the molecular mechanism of cell apoptosis induced by TiO2 NPs.