The low molecular weight acrylic acid was synthesized by precipitation polymerization in petroleum ether utilizing azo-bis-isobutyronitrile (AIBN) as a free radical initiator and isopropanol as chain transfer agent. The influences of monomer concentration, initiator and chain transfer agent amount on the yield of polyacrylic acid are investigated. And the results showed the optimal polymerization conditions were that: the mass percent of initiator (AIBN) was 10% relative to monomer, the chain transfer agent was 100% relative to monomer, the monomer concentration was 100 g/L, and at 80 ℃ for 4h. Product was precipitated in bottom and the solvent can be removed and recycled. A monomer conversion of 92.6% was achieved after the residue was recycled for 5 cycles under the optimized polymerization conditions. The results of the gel permeation chromatography (GPC) showed that the weight average molecular weight (Mw) of polyacrylic acid was kept at 1000~1300 with minimal change and a polydispersity index (PDI) of 1.92~3.38 was obtained.