Laboratory triaxial tests were carried out in order to understand the stress-strain relationship and strength property of grassroots-reinforced soil (GRS). The change of shear strength indexes of GRS with the quantity of grassroots planted in the soil samples was investigated. The failure modes of reinforced and unreinforced soils were discussed; and also the frictional mechanism between the grassroots and soil in the shearing process was studied. The laboratory test results show that: (1) the strength and the capacity for resisting the deformation of grassroots-reinforced soil are better than those of unreinforced soil; (2) as to the shear strength indexes of GRS

the change of internal friction angle is small compare to that of unreinforced soil (its absolute value of relative rate of change is less than 11 %); but the cohesion is more larger than that of unreinforced soil (the maximal increased multiple is nearly 9); (3) in the certain number of grassroots layers

the value of deviator stress

shear strength and cohesion first increase and then reduce with the increasing of grassroots quantity

i.e. there is an optimal reinforcement quantity existing in GRS in the given number of grassroots layers. The research results are of importance for understanding the mechanism of vegetation protection for slopes and determining the density of grass planting.