This work elucidated the intrinsic mechanism underlying the influence of K2HPO4 on PSA production and molecular weight (MW) stability. Among the different potassium salts mixed with K2HPO4 in the initial medium, those with buffering capacity were favorable for PSA production. In the bioreactor culture with pH control, adding an appropriate concentration of K2HPO4 could enhance PSA production. A dual-phase pH control strategy with ammonia water and KOH could also increase the yield and maintain the MW stability of PSA. Zeta potential test, UV/circular dichroism spectra, and transmission electric microscopy were utilized to explore the configuration of K2HPO4-PSA complex. The results from this study can serve a good basis for the industrial-scale production of PSA with stable MW.