Hydrothermal leaching of spent lithium-ion battery (LIB) cathode materials, Li(Ni, Mn, Co)O2 (NMC), was performed with citric acid using a batch-type device and a flow system in turn. Before the hydrothermal experiments, NMC cathode materials were pretreated and characterized. The effect of citric acid concentration on leaching efficiencies of metal components was investigated using the batch-type device. The preliminary experimental conditions for the continuous hydrothermal leaching using the flow system were assessed and pre-set, including the temperature of 200°C, the citric acid concentration of 0.4 mol/L, the pulp density of 10 g/L, and the flow rate of 30 mL/min. NMC cathode materials were leached continuously in hydrothermal water using the flow system. The leachate with a high leaching efficiency could be collected continuously since 65 min of feeding the slurry, and the maximum leaching efficiency of Li, Co, Ni, and Mn was 96%, 91%, 98%, and 94%, respectively.