Systems and methods for synthesizing long-chain nucleic acids molecules are disclosed. The systems and methods described in this application use a Ligation-Purification-Amplification (“LPA”) technique. The LPA technique requires first producing nucleotide sequences with the length of at least 500 bp-1 kbp by assembling smaller oligonucleotide fragments (30 bp-200 bp). The assembled nucleotide sequences are then purified and amplified. This technology can be used to achieve parallel amplification of three or more nucleotide sequences at the same time. In embodiments of the invention, a solid-phase ligation-purification method is adopted, in which nucleic acid molecules are fixed on the surface of beads or other solid particles in order to rapidly purify products obtained from the ligation reaction.