Pyrolysis is a promising technology for the disposal of sewage sludge. In this study, FeCl₃ was selected as an additive for sludge pyrolysis. The results indicated that the FeCl₃-addition strategy not only enhanced carbon retention but also enhanced the nitrogen retention of sludge-based biochar. The best enhancement effect both occurred at 500°C, and the enhanced amount reached 29.7 mg C/g sludge and 1.33 mg N/g sludge, respectively. This enhancement may be attributed to the physical isolation provided by newly formed metallic complexes or oxides. Besides, the added FeCl₃ improved the polarity and aromaticity of modified biochar by retaining more oxygen-containing functional groups, and could also catalyze the decomposition of tar, resulting in the release of more small molecular substances. The quantitative estimation of carbon and nitrogen retention in provinces of China found that the enhancement in coastal provinces was significantly preceded that of inland provinces.