Revealing the evolution law of soil mechanical characteristics under wetting-drying cycles is highly significant for preventing the disaster of expansive soil. In this study, a method of super-mini-penetration (SMP) test for describing soil internal mechanical characteristics is proposed. A series of penetration tests is conducted on the remolded expansive soil specimens under cyclic wetting-drying conditions. The evolution law of penetration resistance with penetration depth and moisture content during three sequences of drying process are obtained. The results show that the proposed SMP test is a simple, quick and effective method to describe the spatiotemporal evolution characteristics of mechanical properties of expansive soil as subjected to drying-wetting cycles. The penetration resistance of the specimen generally increases with increasing drying time. Especially, the penetration resistance of the surface soil relative to the deep soil is more sensitive to drying. The effect of wetting-drying cycles on the spatiotemporal evolution characteristics of the mechanical properties of expansive soil is significant. With an increase in cycles, the penetration resistance of expansive soil decreases. The penetration curves gradually convert from typical mono-peak structure to multi-peak structure; and the spatial difference of penetration resistance becomes more evident, which is more obvious in the relatively low moisture content range. In addition, the relevant mechanisms of the penetration mechanical behaviour of expansive soil under drying and wetting-drying cycles are analyzed by drawing knowledge from soil mechanics and soil structure. During drying, the increase of the penetration resistance of expansive soil is mainly due to the decrease of porosity, the increase of soil density, inter-particle contacts and soil suction induced by shrinkage. The loose structure and cracks resulted by wetting-drying cycles are the main reasons responsible for the weakening of the overall mechanical properties of expansive soil.