The invention discloses a method for preparing magnesium hydrate fire retardant through surface modification treatment by using a composite wet method. The method comprises the following steps: firstly, mixing a water solution of a soluble magnesium salt with an alkali water solution according to a chemical stoichiometric ratio so as to obtain a magnesium hydrate precipitate sizing agent, aging or performing hydro-thermal treatment on the obtained magnesium hydrate precipitate sizing agent, controlling the temperature to be 40-180 DEG C, subsequently performing hyperdispersant and fatty acid (or sodium / or sylvite) compounding wet method surface wrapping modification treatment on magnesium hydrate, and finally dehydrating, drying, crushing and grading the wrapped sizing agent. The powder modified by using the method is good in processing property, excellent and remarkable inflaming retarding and smoke inhibition function, and is applicable to situation of different demands. The method is reasonable in process, low in cost, wide in product application field, large in market potential, high in competitiveness and has remarkable economic and social benefits.