The invention discloses a method for removing magnesium from an ultralow-quality phosphate rock and producing magnesium hydroxide. The method comprises the steps as follows: mixing phosphate rock powder with a magnesium remover for magnesium removal reaction, carrying out solid-liquid separation, obtaining solid-phosphate concentrate, and adding a neutralization and sedimentation agent into liquid to obtain a magnesium byproduct. The method is characterized by taking the ultralow-quality phosphate rock as a raw material and mixed acid of sulphurous acid, sulfuric acid and fluorosilicic acid as the magnesium remover, carrying out solid-liquid separation, mixing washing water for washing a solid phase with a solution obtained after separation, and carrying out neutralization and sedimentation reaction. According to the method, mixed acid of sulphurous acid, sulfuric acid and fluorosilicic acid is adopted as the magnesium remover, so that the P2O5 loss is reduced; the process route is short, the operation is simple and the one-time investment cost is low; the magnesium removal rate can reach more than 80% and the content of magnesium in the obtained concentrate is lower than 1%; and the obtained phosphate concentrate is good in quality, the quality is improved by 5-10%, the P2O5 yield is high and can reach more than 99% under the optimal process conditions, and the P2O5 content is increased by 3-5% in general.