Heavy metal pollution is a significant obstacle to the resourceful application of sewage sludge. The existing research on co-hydrothermal carbonization of sewage sludge and biomass bottom ash is limited. This study examines the use of biomass bottom ash as a modifier at different weight percentages (0, 25, 50, and 75%) to stabilize heavy metals during co-hydrothermal carbonization. The characteristics of heavy metals, including total content, speciation, ecological risk assessment, and leaching content, were thoroughly investigated to ensure the safe use of hydrochars. The results indicate that heavy metals mostly remained in the solid phase, particularly Cr, Ni, Cu, Zn, and Cd. In addition, heavy metals exhibited more stability, and their chemical speciation changed from directly available to potentially active and inactive states in hydrochars. Furthermore, the pH and specific surface area of hydrochars increased slightly. Characterization results and FTIR spectra demonstrated that the aromatization degree of hydrochars significantly increased with the decrease in N/C and H/C ratio. Risk assessment showed that the danger of hydrochars reduced significantly compared to raw sludge. The findings present an innovative method for the collaborative treatment of sewage sludge and biomass bottom ash to prevent heavy metal pollution.Graphical abstract: