A novel approach was used to prepare sewage sludge (SS)-derived biochar via coupling of hydrothermal pretreatment with pyrolysis (HTP) process at 300–700 °C. The influence of the pyrolysis temperature on the characteristics and environmental risk of heavy metals (HMs) in biochar derived from SS were investigated. The HTP process at higher pyrolysis temperature (≥500 °C) resulting in a higher quality of SS-derived biochar and in HMs of lower toxicity and environmental risk, compared with direct SS pyrolysis. Surface characterization and micromorphology analysis indicate that the N2 adsorption capacity and BET surface area in biochar (SRC₂₂₀-500) obtained from hydrothermally treated SS at 220 °C (SR₂₂₀) pyrolysis at 500 °C, significantly increased the BET surface area and achieved its maximum value (47.04 m²/g). Moreover, the HTP process can promote the HMs in SS be transformed from bioavailable fractions to more stable fractions. This increases with the pyrolysis temperature, resulting in a remarkable reduction in the potential environmental risk of HMs from the biochar obtained from the HTP process.