VOPO4·2H2O, as a new cathode material for Ca-ion batteries, exhibits a discharge capacity of 100.6 mA h g-1, excellent cycling stability (200 cycles) and good rate performance (42.7 mA h g-1 at 200 mA g-1). In situ X-ray diffraction and in situ Raman results demonstrate the calcium-ion-storage mechanism of VOPO4·2H2O is a single-phase reaction based on asymmetric Ca2+ insertion and deinsertion.