Characterizing atom/ion mobility in superionic materials is one of the most essential topics. In this work, we show that the copper ion mobility in a promising thermoelectric material CuxAgSey (x, y = 0.98, 1.02), can be significantly changed by modulating the element ratio, x/y. With Cu-63 NMR linewidth as a direct measure of the copper ion mobility, we observed distinguishable Cu ion mobility in the samples with different ratios even under ambient temperatures. This distinction was further evaluated in terms of correlation times and activation energy of copper diffusion. The contrast in ion mobility could be understood from the point of view of vacancy mediation. This work gives fruitful information for understanding ion dynamics and related topics.