The invention provides a cadmium fixing agent for treating contaminated soil and a treatment method of the soil. The cadmium fixing agent comprises lime, charcoal and a plant organic fertilizer; in the aspect of weight, the addition of the lime is 3.7-4.6% of the contaminated soil to be treated, the addition of the charcoal is 2.3-6.9% of the contaminated soil to be treated and the addition of the plant organic fertilizer is 1.0-4.0% of the contaminated soil to be treated; and the content of the effective-state cadmium in the contaminated soil is 6-7%. According to the cadmium fixing agent provided by the invention, the content of the effective-state cadmium in the cadmium fixing agent can be reduced from 6.6mg/kg to 3.1mg/kg. The effective-state content of the heavy metals in the contaminated soil which is treated is reduced to a range that the plants can bear, and cadmium hyper-accumulated plants can be selected for sequential restoration. Another selectable restoration form is provided to the treatment of the heavy metal contaminated soil.