WWTPs (Wastewater Treatment Plants) produce a large amount of daily primary sludge. The composition of primary sludge is complex. The inorganic content is high while the settleability is poor. Isoelectric point pretreatment can effectively destroy the flocculation state of the primary sludge. Free nitrous acid can effectively destruct bacterial structure and promote the hydrolysis of organic matter, the pre-treated primary sludge is more conducive to its fermentation, recycling, final treatment and disposal. The effect of isoelectric point pretreatment by free nitrous acid on primary sludge was studied in this study. Experimental results showed that the settling property of primary sludge is improved. The CST reduced from 203.10 s to 101.65 s. Organic matter is retained to the maximum extent in the solid phase. The COD in supernatant decreased from 1 246.59 mg/L to 1 048.80 mg/L. The metal pollutants in the sludge were released efficiently. The secondary pollution in transport and disposal was prevented and reduced. However, this condition also promoted the dissolution of nitrogen and phosphorus pollutants after 300 mg/L NO₂^--N was added for pretreatment, SCOD dissolution increased, but the release of nitrogen and phosphorus pollutants was significantly reduced. After pretreatment, NH₄⁺-N was only 118.88 mg /L and PO₄^3--P was 6.91 mg /L. These results showed that the best pretreatment condition for primary sludge is 300 mg/L NO₂^--N for 1 h.