The construction of roads in saline soil areas usually involves using coarse-grained soil as roadbed fill material; studying the water–vapor–salt migration mechanism in coarse-grained saline soil subgrades is crucial for ensuring the stability of highway infrastructure. In order to clarify the influence of fines content and initial moisture on the water–salt migration and to clarify the water–vapor–salt migration patterns in coarse-grained saline soil, a model test of coarse-grained saline soil was conducted to study the response patterns of external water replenishment, final moisture content, final salt content, and liquid level height of coarse-grained saline soil. The results indicated that the water vapor migration amount only causes a change in the final moisture content, albeit not enough to cause salt redistribution. With increasing initial moisture content in coarse-grained saline soil, the migration characteristics of water vapor are weakened, and it imposes a significant inhibitory effect on liquid water migration at the same time. Increasing fines content in coarse-grained soil significantly inhibits water vapor migration, whereas liquid water migration is promoted. Water and salt accumulate in the liquid and vapor coupling migration mode at different heights. Based on the mechanisms of water vapor and salt transport characteristics, this study proposes a novel roadbed structure, which is vital for ensuring the long-term service performance of coarse-grained saline soil roadbeds in saline soil areas.