The invention discloses a process for producing in-situ modified nano-magnesium hydroxide by taking phosphate tailings as raw materials. The process comprises the following steps: putting the phosphate tailings obtained by flotation into an acidolysis reactor, reacting for a certain period of time, and then filtering; adding filtrate into an impurity removal reactor, firstly adding an oxidant for oxidation, further adding an alkaline compound, controlling the pH value, filtering a precipitate, then adding the filtrate, a silane coupling agent and a dispersant into an in-situ modification reactor, stirring for a period of time, then adding into a crystallization reactor, introducing ammonia gas, and performing crystallization and precipitation reaction to obtain surface-modified nano-magnesium hydroxide; and producing the filtrate which is rich in free ammonia after crystallization and filtration, obtaining the ammonia gas by air stripping, returning the ammonia gas into the crystallization working section, crystallizing a liquid phase to remove a large amount of ammonium sulfate, and then circulating to the acidolysis reaction process step, thereby realizing closed-loop circulation of wastewater of the process. Surface-modified nano-magnesium hydroxide produced by an in-situ modification method has good interfacial adhesion property and dispersivity in a polymer matrix and can be widely applied to the fields of plastics, coatings, environmental friendliness and the like.