Abstract: With the rapid progress of urbanization, the continuous increase in the generation of municipal sewage sludge and food waste has created considerable challenges for sustainable waste management systems. Heavy metals in municipal sewage sludge, though at low concentration, persist in the environment and bioaccumulate in soil and biota, restricting the safe agricultural use of sewage sludge products due to long-term ecological and health risks. This study aimed to enhance the efficiency of sewage sludge treatment and reduce environmental risks by applying the bio-conversion technology of black soldier fly (Hermetia illucens) larvae (BSFL). Four organic waste types—kitchen waste, fruit and vegetable residues, soybean pulp, and wheat bran—were respectively mixed with sewage sludge at a 1:1 ratio to enrich the nutrient-deficient substrates and enhance BSFL growth and bio-conversion performance. BSFL growth and bio-conversion were assessed via waste reduction rate, biological conversion rate, and feed conversion rate. Heavy metal elements and soil nutrient elements were quantified, and compost safety was evaluated in BSFL and frass based on residual heavy metals levels. The waste reduction rates observed across the four treatment groups ranged from 42.44 to 74.48%, indicating substantial substrate degradation during the BSFL bio-conversion process. Compared with all the initial mixed substrate, the BSFL frass exhibited notable decreases in organic matter content and carbon-to-nitrogen ratio, while the total nutrient content, expressed as the sum of total nitrogen, total phosphorus, and total potassium, increased to 5.35～6.31%, enhancing its potential as an agricultural amendment. Differert substrate composition significantly affected BSFL growth performance, biomass yield, and conversion efficiency, which in turn influenced BSFL frass physical and chemical characteristics and nutrient content. Among the tested combinations, the wheat bran-sewage sludge mixture provided the most favorable conditions for BSFL development, resulting in superior larvae biomass accumulation, waste reduction rate, and other performance. Heavy metals migration analysis revealed that BSFL modulate metals behavior in the substrate through processes such as active absorption, selective retention, physiological regulation, and excretion. The bioaccumulation coefficients of heavy metals differed by BSFL, with cadmium (Cd) exhibiting the highest enrichment, followed by Chromium (Cr), lead (Pb), and arsenic (As), in the order showing as: Cd > Cr > Pb > As. Despite different heavy metals uptake, the concentrations of Cr, Cd, Pb, As, zinc (Zn), and copper (Cu) in the final BSFL frass were significantly lower than those in the initial substrates, indicating effective heavy metals removal during bio-conversion. However, the BSFL frass remained rich in essential soil nutrient elements such as calcium (Ca), magnesium (Mg), manganese (Mn), and sodium (Na), demonstrating its promising potential for agricultural applications. Meanwhile, the harvested BSFL showed good potential for resource utilization as animal feed, and the frass, following appropriate safety assessment, could be safely applied in agricultural land such as cultivated land, garden plots and grassland. This study demonstrated significant improvement in the treatment efficiency of municipal sewage sludge and kitchen waste, fruit and vegetable residues, soybean pulp, and wheat bran via BSFL bio-conversion. The findings offer valuable theoretical and practical insights for the resource recovery and sustainable utilization of food waste-sewage sludge mixed substrates. Furthermore, the research supports the safe application of the resulting compost products and contributes to mitigating environmental risks, thereby promoting the advancement of eco-friendly organic waste management technologies.