To investigate the grammatical constraints of code-switching (CS hereafter) under the disputes of "the constraint-based account" versus "the constraint-free account," the effects of functional category on CS have long been investigated in the existing studies. Thus, the present study, by asking 47 participants to take part in an eye-movement experiment, examined the potential effects of functional category on Chinese-English CS. We found that differential switch costs at varying code-switched conditions as well as robust switch effects that last from the early to the late stage. The findings could tentatively give rise to the theoretical predictions of "the minimalist program," a representative of "the constraint-free account" rather than "the functional head constraint," a typical representative of "the constraint-based account." Moreover, such switch effects might initiate from the early to the very late stage in terms of "time-course of CS processing."