The invention discloses a method for synthesizing magnesia-alumina spinel by roasting chloride ion type Mg-Al hydrotalcite at a low temperature. Raw materials comprise magnesium chloride, aluminium chloride, sodium chloride and sodium hydroxide, wherein the magnesium chloride and the aluminium chloride are used as a mixed salt solution, the sodium chloride solution is used as a base solution, andthe sodium hydroxide is used as a precipitator; and the hydrotalcite is synthesized by a double-titration coprecipitation method to serve as a roasting precursor or pillared hydrotalcite is synthesized in situ by the use of a roasting reduction technology to serve as the roasting precursor. In the process of MgAl2O4 powder preparation, the crystallinity of MgAl2O4 phase is optimized and the impurity content of products is reduced by changing proportions of magnetism-aluminium ions, increasing reconfiguration times, prolonging the time of roasting and thermal insulation, increasing the roasting temperature and the like. The method has simple preparation process and low preparation cost and has a great value for further optimizing the roasting and synthesis of the spinel at a low temperature and the exploration of spinel applied to industrial production.