Embarked from the component and structure of phosphate laser glass, its chemical-mechanical polishing (CMP) mechanism is analyzed. Through experiment, it is demonstrated that the phosphate laser glass has strong selectivity to polishing solution pH value and has high polishing efficiency under slight acidic or neutral condition. By adding pH value adjustment additive in the polishing slurry, the acidity of polishing environment is maintained, as well as the polishing efficiency and polishing quality. An ultra smooth surface of phosphate laser glass with root-mean-square (RMS) 0.6 nm is obtained, through controlling the pH value and density of polishing solution.