To investigate the improvement role of vegetation in the expansive soil, the vegetation was imitated by fine cordage, and the dry shrinkage cracking tests of soil were conducted with different diameter and layout of roots. The crack suppression mechanism of horizontal roots, vertical roots, oblique roots, mixed roots and different root diameter on the expansive soil was analyzed. The statistics demonstrate that the inhibition to swelling-shrinkage deformation from top to bottom is mixed root, horizontal root, oblique root and vertical root. The surface crack rate of reinforcement on expansive soil shows an increasing amelioration trend firstly. The surface crack rate with root diameter of 2 mm group is 1.6%. Root diameter of 1, 3 and 4 mm groups were 2.6%, 3.4% and 4.06%, respectively. The achievement can provide a certain reference for the related research of expansive soil slope ecological reinforcement slope protection.