Abstract: To investigate the effects of different types of biochar co-composting with earthworms on the bioavailability and safety risks of heavy metals (Cu, Zn, Pb, Ni) in industrial sludge, three types of biochar (rice husk charcoal, bamboo charcoal, straw charcoal) and earthworms were added to industrial sludge for composting. The effects of different types of biochar co-composting with earthworms on the physical and chemical properties of sludge, as well as the total amount, availability, morphology, and environmental risks of heavy metals were analyzed. The results showed that compared to composting sludge alone, vermicomposting can lower the pH and OM, and increase TN, TP and EC. Three types of biochar collaborated with earthworms to further reduce the pH of sludge OM, and increase TN, TP, EC. The addition of 4% bamboo charcoal had the best effect. Compared with sludge compost and earthworm compost, biochar combined with earthworm compost significantly reduced heavy metal content and bioavailability. 4% rice husk charcoal combined with earthworm compost had better removal effects on Ni, Cu and Zn, with a total reduction of 31.71%, 13.51% and 25.89% (P<0.05) compared to the control group (T0), and an effective state reduction of 42.80%, 23.27% and 11.06% (P<0.05), respectively. Morphological analysis showed that biochar combined with earthworm compost transformed Cu into organic bound and residual states, while Pb, Ni and Zn transformed into iron manganese oxide bound and residual states. Among them, 4% bamboo charcoal combined with earthworm compost had the highest conversion rate and better passivation effect. The potential ecological risks of three types of biochar combined with earthworms for sludge treatment were all low: rice husk charcoal combined with earthworms had the lowest risk (RI of 95.9), while straw charcoal combined with earthworms had a relatively higher risk (RI = 97.04). The health risk assessment found that the total non-carcinogenic risk (HI) of non-carcinogenic substances to adults and children through oral intake, skin contact, and respiratory intake in the treatment group was less than 1, which was at an acceptable level for the human body. The total risk of straw charcoal combined with earthworms was the lowest (HI = 0.092). In summary, the combination of biochar and earthworm composting provides an effective strategy for reducing heavy metal content and ecological risks in sludge. Bamboo charcoal is better at improving physical and chemical properties and passivating heavy metals, while rice husk charcoal is superior to bamboo charcoal and straw charcoal in terms of total heavy metal removal and safety. Straw charcoal performs well in overall risk control. All three have demonstrated good application potential.