The invention relates to a synthesis method of a high-molecular-weight polyaspartic acid, particularly a polyaspartic acid synthesized by a microwave half-solvent method. The invention aims to overcome the defects of low molecular weight of the product, unstable antiprecipitation property, difficulty in solvent separation and the like in the existing microwave synthesis method. The synthetic process comprises the following two steps: 1. by using maleic anhydride or fumaric acid and ammonia water or ammonium salt as raw materials, adding a small amount of solvent, and radiating with microwaves, of which the frequency is 915+/-50 MHz or 2450+/-50 MHz and the power is 200-20000W, for 1-40 minutes to synthesize the low-molecular-weight polysuccinimide (PSI-I), wherein the solvent is recycled completely; and 2. by using the PSI-I as the raw material, radiating with microwaves, of which the frequency is 915+/-50 MHz or 2450+/-50 MHz and the power is 400-50000W, under the condition of no solvent for 1-30 minutes to synthesize the high-molecular-weight polysuccinimide (PSI-II), and hydrolyzing the PSI-II to obtain the high-molecular-weight polyaspartic acid. The invention has the advantages of simple technique, no need of solvent separation, obviously higher product molecular weight, obviously higher antiprecipitation property, high reaction speed, high yield, energy saving, low pollution and the like.