The invention discloses a method for synthesizing ceftriaxone sodium. The method includes the steps that a compound I and a compound II are added into a reaction vessel containing dimethyl carbonate, a stirring reaction is carried out for 1-2 h at 30-40 DEG C, then a compound III is added, PEG-800 is added with stirring, triethylamine is added after stirring is carried out for 10-20 min, a stirring reaction is carried out for 3-4 h at 8-10 DEG C, then a sodium hydroxide solution is dripped till the pH is 7, excessive acetone is added, white crystals are separated out, and a compound V is obtained after drying. According to the method, 7-ACA, sulfo-triazine heterocycle and cefotaxime side chains are subjected to a one-pot reaction, and successive condensation is carried out without separation; no expensive agent is used in the reaction process, environment-friendly low-toxicity dimethyl carbonate is adopted as a reaction solvent, and in a homogeneous-phase environment formed by PEG-800, high-purity ceftriaxone sodium is obtained at a high yield through simple steps; in addition, the magnification effect is effectively avoided, and the method is particularly suitable for industrial production.