Pseudocapacitors, which can store more energy at high charge/discharge rates, have attracted considerable attention. The performance of a pseudocapacitive material mainly depends on the interaction between electrode materials and the electrolyte ions. However, the understanding of the interaction is still limited. Here, the performance of Ti2CT2 (T = O, F, and OH) nanosheets as pseudocapacitor electrode materials has been investigated through a novel first-principles approach. The results suggest that O-terminated Ti2C nanosheets are shown to be pseudocapacitive cathode materials. The pseudocapacitance is attributed to the large intrinsic capacitance of Ti2CO2 nanosheets and contact adsorbed cations. The former mainly decides the capacity of charges and the latter reduces the change of potential in the electrode. The integral capacitance and Na-ion capacity of Ti2CO2 nanosheets are simulated to be 291.5 F g(-1) with a broad potential window range from 0 to 2.80 V (versus Na/Na(+)). Low diffusion energy barriers on Ti2CO2 and Ti2CF2 nanosheets indicate fast transportation and high charge and discharge rate for Na-ions. Our results provide insight into the origin of pseudocapacitance on stacked two-dimensional materials.