In order to ensure the performance of semi-rigid subgrade under the premise of maximum use of steel slag, naturally graded cement stabilized steel slag mix with poor gradation was applied to low-grade road subgrade to explore the influence of gradation on the mechanical properties, shrinkage properties and durability of cement stabilized steel slag mix. Moreover, the feasibility and applicability of this mix in the construction of semi-rigid subgrade of low-grade roads were investigated. The results showed that the mechanical properties of the four types of graded cement stabilized steel slag mixes increased with the increase of age and cement dosage, with the best performance of the mix with medium gradation and the worst performance of that with natural gradation. The gradation significantly affects the mechanical properties, shrinkage properties and durability properties of the mix. Although the road preformance of naturally graded cement stabilized steel slag mix is inferior due to poor gradation, its road performance indicators are still able to meet the requirements of the specification for low-grade roads, as well as maximizes the use of steel slag in the construction of road subgrade, which will bring better social and economic benefits. Therefore, it is recommended that the cement stabilized steel slag mix with natural graded steel slag should be adopted for the low-grade road subgrade.