The interface structure induced by electrolyte cations was found to play a significant role in determining the performance and stability of dye-sensitized solar cells. The trap state density in the nanostructure TiO2 electrodes was affected by the adsorbed 1,2-dimethyl-3-propylimidazolium cation (DMPI+) or alkali cations, such as Li+, Na+, K+, and Cs+, on the dyed TiO2 electrode and was found to increase with the order of decreasing cation radius DMPI+ < Cs+ < K+ < Na+ < Li+. The change in interface structure resulted from the accumulation of the adsorbed cations to increase trap states in the nanostructure TiO2 electrodes during long-term accelerated aging tests. The size effect of electrolyte cations on the cell performance suggested that the reduced surface cations, when small cations penetrated into titania lattice, resulted in a negative shift of the TiO2 conduction band edge and a weaker interaction of Li+ with dyes to obtain the decline in photocurrent and efficiency. The overall efficiency of dye-sensitized solar cells with large DMPI+ in the electrolyte retained over 110% of its initial value after 2100 h. Also, no obvious differences in the efficiency for dye-sensitized solar cells with electrolyte cations, such as Li+, Cs+, and DMPI+, were observed after 1270 h under one sun light soaking in our experiment. The results suggested that large DMPI+ chemisorbed on TiO2 surface could not intercalate into the TiO2 lattice for the enhanced stability of dye-sensitized solar cells in practical application.