Bilinguals have shown an advantage over monolinguals in cognitive functioning. Likely due to their experience of language switching, bilinguals tend to perform better in task switching. Support for such an advantage effect has been obtained by using the task-cueing paradigm. The paradigm, however, may confuse cue-change with task-change. Therefore researchers have developed a modified task-switching paradigm with a 2:1 mapping between cues and tasks. Using this modified paradigm, the present research investigates brain functioning of proficient and non-proficient bilinguals in task switching, and explores whether endogenous preparation accounts for the bilingual advantage in task switching. Two experiments were reported in the current paper. In a behavioral experiment, by using a task-switching paradigm with a 2:1 mapping between cues and tasks, we explored whether proficient bilinguals who constantly switch between two languages in daily life would show advantage over non-proficient bilinguals due to the endogenous preparation of task switching. This experiment, conducted with E-prime, included 30 proficient bilingual and 30 non-proficient bilingual university students. In an ERP experiment involving 11 proficient bilinguals and 11 non-proficient bilinguals, we examined brain activities by observing the wave of D-Pos, as an index of endogenous preparation. The data collection and analyses were conducted with Vision recorder and analyzer Software produced by Brain Products corporation. The experiments found that the reaction time was faster for proficient bilinguals than for non-proficient bilinguals in both task-change and task-repeat sequences, when the cue-stimulus interval was 250 ms, the reaction time and the switching costs were smaller for proficient bilinguals than for non-proficient bilinguals. Most importantly, we observed stronger RISC (Reduction In Switch Cost) effects among proficient bilinguals than among non-proficient bilinguals when the time interval was from 0ms to 250ms. Used event-related potential, we carried out a further study on time-course features of endogenous preparation with D-Pos as the index. According to the analysis of cue-locked D-Pos component, non-proficient bilinguals presented a more positive component than the proficient bilinguals. We conclude that the experience of using two languages in proficient bilingual improves cognition processing in general, and enhances executive functioning in particular.