In　this　article　we　study　quasi-neutral　limit　and　the　initial　layer　problem　of　the　drift-diffusion　model.　Different　from　others　studies,　we　consider　the　physical　case　that　the　mobilities　of　the　charges　are　different.　The　quasi-neutral　limit　with　an　initial　layer　structure　is　rigorously　proved　by　using　the　weighted　energy　method　coupled　with　multi-scaling　asymptotic　expansions.