The influence of phosphate ions on the pitting corrosion of stainless steel type 316L was investigated with electrochemical and thermo-chemical methods. DC polarization measurements showed that the pitting could occur spontaneously above 72 °C on SS-316L in chloride containing simulated coal gasification waste water (CGW). Addition of phosphate ions above critical pitting temperature (CPT) in the simulated CGW retarded the oxygen reduction reactions, reduced the pitting frequency and impeded the pitting growth considerably. EIS study revealed that the PO43− ions enhanced the pitting resistance of SS-316L by absorbing and modifying the passive film through the interaction with the hydrated Fe species. XRD analysis of SS-316L surface exposed in simulated CGW containing PO43− ions at 170 °C further confirmed the existence of iron phosphate in the surface corrosion layer.