The invention relates to a pretreatment method and device for improving the washing and desorption performance of soil. The pretreatment method comprises the following steps that vortex desorption pretreatment is completed through the combination of a jacket type spiral feeding screen and a hydrocyclone; the screened polluted soil is stirred in stirring tanks, an eluting agent is added, and/or ultrasonic equipment is adopted for cleaning; and after sufficient mixing and reaction, the soil enters the jacket type spiral feeding screen inclining by a certain angle, large particles are screened in a cylinder under spiral driving, middle and small particles penetrate through a screen, flow downwards along with water or a mixed solution of water and the eluting agent and return to the stirring tanks, then under the separation action of the hydrocyclone, small particles are discharged out of an overflow port, middle-sized particles are discharged out of an underflow port, and therefore classification of the large particles, the middle-sized particles and the small particles is achieved. Differential treatment is achieved, the vortex desorption performance of the soil is improved, and the pretreatment method is excellent. The pretreatment method has the advantages of being simple, convenient, efficient, capable of saving energy, fast, small in side effect, and free of secondary pollution. Parameters of the pretreatment device can be adjusted and optimized according to actual conditions, pollutants containing heavy metal, pesticides and the like are effectively removed, and the defects of the industrialized technology of current soil washing and desorption pretreatment are overcome.