By means of triaxial shearing test in laboratory, effect of the interaction of soil water content and bulk density on soil shearing strength in yellow soil, calcareous purple soil and neutral purple soil was explored. The three types of soils are extensively distributed in the hilly regions of Chongqing. Dynamics of the soil shearing strengths of the three types of soils under wetting-drying alternation were analyzed, using soil samples adjusted to be optimal in soil water content and bulk density specific of the three soils, separately, based on the analysis of effect of soil water content and bulk density on soil shearing strength. Results show that the soil cohesive force first increased and then decreased with the increasing soil water content in all the three soils unchanged in soil bulk density, and it increased with the increasing soil bulk density in the soils the same in soil water content. Internal frictional angle decreased significantly with the increasing soil water content in all the three soils, regardless of bulk density. The interaction of soil water content and bulk density affected soil cohesive force significantly, which increased with increasing soil bulk density in the range of 1.3~1.7 g cm^-3, and would peak when bulk density interacted with a matching soil water content. The interaction did not have much effect on internal frictional angle, which did not vary much when bulk density remained the same, but increased gradually with increasing soil bulk density. In all the three soils, soil cohesive force decreased with increasing number of wetting-drying alternations, and it dropped sharply in the first 2 cycles of wetting and drying and very slowly during the following three cycles, and then leveled off. Internal frictional angle as a whole decreased with increasing number of wetting-drying alternations in all the three soils, however, differences were found between the soils different in soil type. The angle was 24.6°, 22.6° and 19.3° in yellow soil, neutral purple soil and calcareous purple soil, respectively after the fifth cycle of wetting-drying alternation.