Karst rocky desertification in the southwest of China has seriously influenced local people's life. Essentially, soil erosion is the key causing rocky desertification. The red clay, which widely distribute in karst region, is more easily to be eroded under the rainfall, because it's shear strength is changeable with water content. This paper presents a comparative analysis of direct shear test for undisturbed and remolded soil samples (modeling the reclaimed soil). The analysis preliminarily reveals the laws of the shear strength of red clay changing with water content. It is found that whenits water content 35%, the the cohesion and internal friction angle of the red clay decrease with the increasing of water content. The shear strength of undisturbed soil is significantly larger than that of the remolded soil under the same water suction. As water content increases, the shear strength of the remolded soil samples attenuates more drastically than that of the undisturbed soil. It shows that the reclaimed red clay is more easily to be eroded under the rainfall.