<p>The shear strength parameters serve as pivotal mechanical indices governing both subgrade stability and pavement structural durability in highway engineering. This study investigates the correlations between shear strength parameters of compacted carbonaceous shale soils and moisture content, compaction degree, and freeze–thaw cycles through direct shear tests. The effects of these factors and their interactions on shear strength parameters were systematically analyzed. The results indicate that the cohesion of the soil initially increases and then decreases with increasing moisture content. Meanwhile, the internal friction angle decreases with increasing moisture content. Furthermore, the lower the compaction degree, the greater the reduction in the internal friction angle. With increasing compaction degree, both cohesion and internal friction angle increase. Additionally, the higher the moisture content, the more pronounced the influence of compaction degree on the internal friction angle, while its effect on cohesion gradually weakens. A moisture content of 22% can be regarded as the threshold value that delineates the mechanism by which compaction degree enhances the shear strength of carbonaceous shale soils. Cohesion decreases progressively with freeze–thaw cycling and stabilizes after 5 cycles. A greater reduction is observed at higher moisture contents and lower compaction degrees. In contrast, the internal friction angle demonstrates minimal variation under freeze–thaw cycling. A multivariate regression model for shear strength parameters under coupled influencing factors was developed. The results provide theoretical basis and technical support for the construction of carbonaceous shale subgrade, optimized pavement structural design, and frost damage prevention in seasonal frozen regions.</p>