Abstract: People have known that the fissure is a remarkable feature of expansive soil, and it has influences on the slope stability of expansive soil. But the mechanism and the extent of the influences are not clearly known. The influences of fissure are not considered in the design and construction of expansive soil slopes, resulting in frequent occurrence of the landslides of expansive soil. It is necessary to thoroughly study this problem. The authors have published the test results of the influences of the fissures on the strength parameters of expansive soil, developed an analysis method for slope stability considering the influences of the fissures, and put forward a reinforcement method using the geo-membrane to restrict the development of the fissures. In this paper, the mechanism and development process of the fissures are discussed, the effect of the fissures in some practical instances of expansive soil landsides is analyzed, the characteristics of landsides of expansive soil which are different from those of other cohesive soils are explained based on the fissures, the success of the replacement method is revealed mainly due to the restriction of the development of the fissures. All of the above points show that the fissure is the key problem of instability of the expansive soil slopes. It is proposed that the influences of the fissures should be considered in the analysis, calculation, construction and treatment of the slopes of expansive soil, and the corresponding systematic theory should be developed.