Polysialic acid (PSA) is a capsular polysaccharide obtained from aerobic fermentation with Escherichia coli. To enhance PSA production and eliminate the influence of phosphate on the PSA purification process, a lower level of initial phosphate was adopted with pH control. The resulting PSA yield reached 4.1 g/L in fed-batch fermentation with 2.5 g/L $K_2HPO_4$ and E. coli strain CCTCC M208088. In addition, an ammonia water ($NH_4OH$) feeding strategy to control the pH at 6.4 was developed resulting in PSA production that reached as high as 5.2 g/L. NMR spectra confirmed the purified biopolymer as a ${alpha}$-2,8 linked PSA, identical to the published NMR spectra, with a molecular weight in the range of 16 ~ 50 kDa.