The invention relates to a metal deactivator used for preventing corrosive sulfur corrosion, and a preparation method and applications thereof. The metal deactivator is a hemiacetal benzoxazole compound with a structure represented by formula I, wherein R1 and R2 are respectively selected from H or Me at random, A is selected from N or CH at random, n is equal to 1 or 2, X is selected from CH2 or O at random. No high toxicity substances including formaldehyde and bis(2-ethylhexyl)amine is released into the environment in application process of the metal deactivator, additive utilization safety is improved, and the metal deactivator can be widely applied in electrical equipment such as power transformers, distribution transformers, or electric reactors.