The invention relates to the field of soil restoration, especially relates to the field of restoration of soil polluted by heavy metals, and particularly relates to a passivation agent for cadmium polluted alkalescent soil and a restoration method. The passivation agent is prepared from montmorillonoid and cow dung. The dosage of the passivation agent is 16%. The passivation agent formed through combination of montmorillonoid and cow dung can remarkably reduce the concentration of effective state Cd in the polluted farmland soil. Meanwhile, a test proves that the passivation agent formed through combination of montmorillonoid and cow dung has a better passivation restoration effect compared with independent application of cow dung or montmorillonoid, and the passivation rate is obviously higher than the sum of the passivation rates of montmorillonoid and cow dung. When the application rate of the passivation agent is 16%, the passivation rate can be as high as 81.48%. Therefore, the passivation agent provides a more effective new route for restoration of the alkalescent farmland soil polluted by the heavy metals.