Municipal sludge was treated by acid-base(pH 1.0, 3.0, 5.0, 7.0, 9.0, 11.0) method and acid-base combined low-temperature(5, 25, 45, 65, 85 ℃) heat treatment method for 24 h. By comparing the changes of the mass concentrations of orthophosphate(PO43--P) in the sludge liquid phase and the mass fractions of total phosphorus(TPs), inorganic phosphorus(IPs) and organic phosphorus(OPs) in the solid phase before and after treatment, the migration law of phosphorus in the sludge during treatment was studied. By comparing the changes of dissolved chemical oxygen demand(SCOD), soluble protein and soluble carbohydrate mass concentrations in sludge liquid phase before and after treatment, the effects of acid-base treatment and acid-base combined low-temperature heat treatment on sludge cracking were studied, and the mechanism of sludge phosphorus release was explored. The results showed that when the pH were 1.0 and 11.0, the phosphorus release rate and cracking degree of sludge treated by acid and alkali for 24 h were the highest in acid treatment and alkali treatment, respectively. The mass concentrations of PO43--P in the liquid phase were increased from 5.87 mg/L before treatment to 217.58, 136.87 mg/L, respectively; the SCOD were increased from 267 mg/L before treatment to 4 369, 6 548 mg/L, respectively; the mass concentrations of soluble carbohydrate increased from 119 mg/L before treatment to 1 649, 2 861 mg/L, respectively; the mass concentrations of soluble protein increased from 138 mg/L before treatment to 1 813, 3 064 mg/L, respectively; the release rates of sludge TPs were 58.03% and 30.05%, respectively; and the release rates of non-apatite inorganic phosphorus(NAIP) were 78.36% and 54.79%, respectively. The release rate of apatite inorganic phosphorus(AP) with pH 1.0 was 95.71% and the mass fraction of OPs increased by 13.11%; while the mass fraction of AP with pH 11.0 increased by 44.21% and the release rate of OPs was 15.42%. It could be concluded that under the condition of acid treatment, the release of sludge phosphorus mainly came from the release of NAIP and AP, while under the condition of alkali treatment, the release of sludge phosphorus mainly came from the release of NAIP. When the pH were 1.0 and 11.0, and the combined heat treatment temperature was 85 ℃ for 24 h, in the sludge liquid phase, the mass concentrations of PO43--P were 323.52, 220.10 mg/L, respectively; SCOD were 6 400, 9 470 mg/L, respectively; the mass concentrations of soluble carbohydrate were 2 696, 5 015 mg/L, respectively; and the mass concentrations of soluble protein were 3 562, 4 258 mg/L, respectively; the release rates of TPs in sludge were 62.67% and 51.32%, respectively and the phosphorus release and sludge cracking degree reached the highest; and he release rates of NAIP were 86.22% and 87.94%, respectively. The release rate of AP at pH 1.0 combined with 85 ℃ was 96.14%, and the mass fraction of OPs increased by 13.62%; while the mass fraction of AP at pH 11.0 combined with 85 ℃ increased by 77.25%, and the release rate of OPs was 40.87%. When the pH were 1.0 and 11.0, and the combined heat treatment temperature was 45 ℃ for 24 h, in the sludge liquid phase, the mass concentrations of PO43--P were 272.16, 184.35 mg/L, respectively; SCOD were 5 960, 9 030 mg/L, respectively; the mass concentrations of soluble carbohydrate were 2 331, 4 699 mg/L, respectively; the mass concentrations of soluble protein were 3 523, 4 246 mg/L, respectively; the release rates of TPs in sludge were 61.12% and 41.07%, respectively; and the release rate of NAIP were 83.21% and 69.97%, respectively. The release rate of AP at pH 1.0 combined with 45 ℃ was 96.14%, the mass fraction of OPs increased by 12.60%; while the mass fraction of AP at pH 11.0 and 45 ℃ increased by 45.92%, and the release rate of OPs was 24.16%. Considering the factors including energy consumption, chemical cost and secondary pollution, we concluded that the pH 1.0 combined with 45 ℃was optimal for sludge treatment.