The invention relates to a method for synthesis of calcium alginic acid monosaccharide from shells and alginic acid. The method comprises shell pre-treatment, alginic acid degradation, synthesis, crystallization, and drying. The method comprises the following steps of 1, immersing shells in hydrochloric acid having the content of 0.05M, carrying out drying, carrying out crushing, carrying out treatment by an elutriation method to obtain calcium-source shell powder, adding alginic acid into an appropriate amount of a H2O2 solution having the content of 3%, carrying out heating degradation by a water-bath heating and microwave heating combined method, wherein in the heating degradation, a water-bath heating process is carried out at a temperature of 80 DEG C for 1h, then a microwave heating process is carried out for 1.5h, then a water-bath heating process is carried out at a temperature of 80 DEG C for 1h and then a microwave heating process is carried out for 1h, and carrying out centrifugation, purification, concentration and drying of the degradation products to obtain white alginic acid monosaccharide, and 2, dissolving the white alginic acid monosaccharide in an appropriate amount of distilled water, adding the calcium-source shell powder into the alginic acid monosaccharide solution according to a mole ratio of the alginic acid monosaccharide to the shell powder of 2: 1.0-1.2, wherein the calcium carbonate content is 95%, carrying out a reaction in a water bath at a temperature of 30 DEG C for 0.75h, carrying out suction filtration, and carrying out reduced pressure concentration, alcohol precipitation crystallization and drying of the supernatant. The method has simple processes. The calcium alginic acid monosaccharide can be used as a novel calcium supplement agent. The method realizes high-value utilization of alginic acid and a plurality of shells.