Abstract: Preparing high-performance magnesium hydroxide from waste bischofite in salt lakes is a current research focus, and achieving precise control over the crystal face polarity of magnesium hydroxide and the chlorine content of the product is key to its application in polymer materials. This study used waste bischofite from salt lakes as the raw material and employed an ammonia-based precipitation method to prepare magnesium hydroxide. It investigated the effects of reaction temperature, stirring rate, ammonia concentration, and reaction time on the magnesium hydroxide products’ facet-selective growth and chlorine content. The product’s crystal form and crystal face characteristics were characterized by X-ray diffraction (XRD) and other analytical techniques. The results showed that reaction conditions significantly impacted product chlorine content and crystal face polarity. Under optimal reaction conditions (65 ℃, stirring rate 700 r/min, ammonia concentration 6.0 mol/L, reaction time 15 min), the chlorine content in the magnesium hydroxide product decreased to 0.30%, and the intensity ratio of characteristic peaks of the (001) and (101) crystal faces reached 2.22. This study provides theoretical support for effective control of facet polarity and chloride content in ammonia-method preparation of magnesium hydroxide from salt-lake magnesium resources.