Thermal desorption has been widely used as an efficient method for remediating Hg-contaminated, whereas the current thermal treatment approach generally needed a high temperature (above 600 degrees C), which negatively affected the soil composition and consumed energy. For the challenge, this study developed a novel Na2S leaching assisting thermal desorption method for thoroughly repairing the severely Hg-polluted (168 mg/kg Hg) soil with a declined temperature (350 degrees C). The results of this study indicate that Hg content could only be reduced from 168 mg/kg to 32.4 mg/kg by directly heating the soil at 550 degrees C for 60 min. However, the Na2S leaching assisting thermal desorption method could realize the low temperature and thorough remediation for highly Hg-polluted soil. When the thermal desorption temperature was reduced to 350 degrees C and the thermal desorption time was kept at 60 min, the Hg content was decreased to 1.0 mg/kg, which was less than the Chinese National Standard. This should be ascribed to the fact that high-temperature volatile Hg compounds in the soil were first removed by Na2S leaching pretreatment. This new collaborative remediation method could not only realize mild thermal desorption but also provide useful guidance for the severely Hg-contaminated soil.