The invention discloses a method for preparing highly pure magnesium hydroxide and magnesium hexa-silicate by using olivine tailing mine, which comprises the following steps: step one. olivine tailing mine and muriatic acid are blended, leached and reacts; step two. pickle liquor leached by acid is added into an enamel reactor and oxidized by adding oxidant hydrogen peroxide; step three. the component of a solid product after reaction is a ferric oxide which is washed and dried after solid-liquid separation to be iron oxide red; step four. filtrate after solid-liquid separation is continouslyadded with ammonia water; step five. sizing agent containing magnesium hydroxide stays in reactant, aged, washed, dried and ground to obtain highly pure magnesium hydroxide; step six. solid leached by muriatic acid reacts with caustic soda to produce serous fluid containing sodium silicate; and step seven. reaction product which reacts with caustic soda solution is filtered and washed to obtain magnesium hexa-silicate. The method causes magnesium oxide to be fully recycled. The method ensures complete circulation of ammonia, increases yield of magnesium oxide in mineral, reduces the cost of magnesium hydroxide, and simultaneously ensures the purity of the magnesium hydroxide which reaches more than 99%.