The invention relates to a catalyst precursor used for preparing conjugated diene or styrene through olefin or ethylbenzene selective oxidative dehydrogenation. The catalyst precursor comprises a hydrotalcite-like substance, as well as an optional ferrite spinel substance and a binder and/or adhesive and/or an assistant. The invention also relates to a catalyst used for preparing conjugated diene or styrene through olefin or ethylbenzene selective oxidative dehydrogenation. The catalyst comprises composite metal oxide, as well as an optional ferrite spinel substance and a binder and/or adhesive and/or an assistant. The invention also relates to a method used for preparing conjugated diene or styrene through olefin or ethylbenzene selective oxidative dehydrogenation. The method comprises the steps: (A) providing olefin and mixed gas of oxygen-containing gas and water vapor or providing ethylbenzene and mixed gas of carbon dioxide and water vapor as reactants; and (B) enabling the reactants to continuously pass through a reactor being loaded with the catalyst precursor or the catalyst for oxidative dehydrogenation reaction to obtain dehydrogenized conjugated diene or styrene.