Transcranial direct current stimulation (tDCS) provides a non-invasive approach to modulate brain functions. Some studies have shown that tDCS combined with working memory training can alter the effect of training. This study aims to investigate the effect of HD-tDCS over the left dorsolateral prefrontal cortex combined with N-back task on the amplitude of event related potentials (ERP). In the experiment, subjects performed N-back training for 30min every day with active or sham tDCS for 10 days. EEG data were recorded when subjects performing N-back tests prior to the training, 1 day and 20 days post the training, respectively. With the analyses of ERP components, it was found that there were no significant differences between active and sham groups. However, the results of post-test were significantly different from the pre-test. Subsequently, both in active group and in sham group, the amplitude of ERP increased in the frontoparietal and occipital regions 1 day post training. Those alterations were enhanced 20 days post training in the active group but not in the sham group. The results indicated the aftereffect of HD-tDCS to promote the effects of cognitive training, showing accumulative positive aftereffects on ERP 20 days after the stimulation.