The invention provides a primer-free gene synthesis method based on a plasmid library, and the method is as follows: constructing a plasmid library containing 16384 plasmids (any combination sequence of four basic groups of 47 and 7bp), starting from a constructed plasmid containing 7 bp, dividing a selected target gene according to each 82bp, successively reconstructing four resistance genes Kana, cam, gem and spc by enzyme digestion and enzyme connection operation to obtain a plurality of 82bp plasmids, and assembling the 82bp plasmids into long fragment genes. According to the method, after construction of the plasmid library containing the 16384 plasmids (any combination sequence of four basic groups of 47 and 7bp), various target genes can be synthesized without any primer. The synthesis method has the advantages of simple operation, extremely low mutation rate, no need of sequencing, low cost and less amount of screening, is conductive to constructing a mutant library, can realize solution operation, and is easy to realize automation.