The invention relates to a harmless disposal method for high-arsenic heavy metal sludge and belongs to the technical field of hazardous waste treatment. The harmless disposal method comprises the following steps of firstly, precalcining the high-arsenic heavy metal sludge with water content of less than 40 percent under the condition that the temperature is 100 to 300DEG C for 0.5 to 3 hours to obtain a calcined product; adding lime and additives into the obtained calcined product according to the mass ratio of (30 to 80): (10 to 40):(1 to 30), and uniformly stirring to obtain a mixed material; forming the obtained mixed material to produce a solidified body, cooling and then safely storing. According to the sludge disposed by the harmless disposal method, leaching concentrations of all heavy metal elements meet the requirements of Identification Standard and Leaching Toxicity Identification of Hazardous Waste (GB5085.3-2007) and Landfill Pollution Control Standard for Hazardous Waste (GB18598-2001).