The invention relates to a method for synthesizing acrylic acid from cyanobacteria. The method comprises the following steps: 1), firstly, based on an NSI gene of cyanobacteria Syn7942, constructing an integrated vector NSI, and recombining to obtain a vector NSI-ceaS2; 2), transforming the recombinant vector NSI-ceaS2 into cyanobacteria Syn7942 cells, and then preliminarily screening out monoclonal transgenic cyanobacteria Syn7942 through a chloramphenicol solid culture medium; 3), transferring the screened monoclonal transgenic cyanobacteria Syn7942 to a liquid culture medium with chloramphenicol resistance, and after the cyanobacteria grow out, extracting a cyanobacteria genome for PCR verification of a target gene; 4), transferring the successfully verified monoclonal transgenic cyanobacteria Syn7942 to the liquid culture medium for culture, and when the cyanobacteria grow till OD730 is greater than or equal to 1, adding IPTG for inducing expression of a gene ceaS2, and catalytically synthesizing the acrylic acid by an enzyme by using glyceraldehyde-3-phosphate (G3P) or dihydroxyacetone phosphate (DHAP) produced by photosynthesis of the cyanobacteria as a substrate; 5), performing separation and purification. By the method, when the acrylic acid is synthesized through transformation of the cyanobacteria as substrate organisms, only sunlight and moisture and the like are required as production raw materials, production equipment and a production environment are easy to construct, and high electricity consumption is not required, so that the production cost of the methodprovided by the invention is greatly lower than those of other production methods.