The invention provides production technology of magnesium alginate. The production technology comprises the following steps: (1) preparing alginic acid from brown algae via alkali digestion; (2) adding alginic acid and a certain amount of composite magnesium salt into a reactor, and reacting for 50-75 min till alginic acid and composite magnesium salt are neutralized and are completely converted to magnesium alginate, wherein the neutralization reaction comprises a solid-phase neutralization reaction and a liquid-phase neutralization reaction taking alcohol as a medium; (3) using the composite magnesium salt with the amount being 7-15% of that of alginic acid; (4) taking magnesium chloride and basic magnesium salt, the ratio of which is 10-20:80-90, to prepare the composite magnesium salt, wherein (5) the basic magnesium salt comprises one or more of magnesium bicarbonate, magnesium hydroxide, magnesium carbonate, magnesium oxide, magnesium acetate, and magnesium silicate; and (6) performing centrifugation, plunger extruding granulation, drying, and pulverizing to prepare magnesium alginate with a required mesh size. Magnesium alginate prepared in the invention has the viscosity in a range of 100-2000 mPa.s, the mesh size in a range of 40-320 mesh, the pH value in a range of 6.0-8.0, a water content no more than 15.0%, and an ash content in a range of 14-28%. The production technology can be realized in a condition of conventional solid-phase, and liquid-phase neutralization production technology of algin.