BackgroundMaintaining homeostasis of Ca2+stores in the endoplasmic reticulum (ER) is crucial for proper Ca2+signaling and key cellular functions. Although Ca2+depletion has been known to cause ER stress which in turn activates the unfolded protein response (UPR), how UPR sensors/transducers respond to excess Ca2+when ER stores are overloaded remain largely unclear.ResultsHere, we report for the first time that overloading of ER Ca2+can directly sensitize the IRE1α-XBP1 axis. The overloaded ER Ca2+in TMCO1-deficient cells can cause BiP dissociation from IRE1α, promote the dimerization and stability of the IRE1α protein, and boost IRE1α activation. Intriguingly, attenuation of the over-activated IRE1α-XBP1s signaling by a IRE1α inhibitor can cause a significant cell death in TMCO1-deficient cells.ConclusionsOur data establish a causal link between excess Ca2+in ER stores and the selective activation of IRE1α-XBP1 axis, underscoring an unexpected role of overload of ER Ca2+in IRE1α activation and in preventing cell death.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13578-023-01062-y.