The invention discloses a thermal desorption device for treating mercury-contaminated soil. The device comprises a feeding unit, a heating unit, an auxiliary exhaust unit, a soil collection unit and awaste gas purification unit; the feeding unit is used for transporting soil crushed by a pretreatment unit to the heating unit; the heating unit is used for heating the soil input by the feeding unit; the auxiliary exhaust unit is used for transporting waste gas discharged from the heating unit to the waste gas purification unit by means of nitrogen gas; the soil collection unit is used for collecting heated and purified soil; the waste gas purification unit is used for purifying and filtering mercury-containing waste gas and discharging; a stirring shaft is arranged in a heat rotary kiln ofthe heating unit for overturning the soil for uniform heating; the mercury-containing waste gas is sent to the waste gas purification unit through a pipeline I and a pipeline II under the push of thenitrogen gas transported by a nitrogen inlet pipeline. On the basis that the device enables the soil to be uniformly heated, the mercury-containing waste gas can be completely discharged, and the discharge speed of the the mercury-containing waste gas can be effectively improved; the entire structure is simple, and the use is convenient.