Over-compaction or under-compaction of soil-rock mixture subgrade could lead to diseases after the expressway is put into service. Selecting the appropriate compaction process and compaction quality testing method is the key to controlling the compaction quality of soil-rock mixture subgrade. Based on continuous compaction control technology, the subgrade porosity under rolling was simulated by two-dimensional discrete element method, considering lay thickness, roller mass, and roller passes. The suitable filler type and recommended compaction process for the soil-rock mixture subgrade in Xingtai Section of Taihangshan Expressway were given: the lay thickness of soil-rock mixture subgrade is 0.6–0.7 m; the mass of the roller is 26 t, and the number of roller passes is rolled 5–6 passes. The evolution law of the subgrade porosity considering the particle crushing was analyzed. A method to control the compaction quality of the soil-rock mixture subgrade was proposed: whether the average difference between the in-site measurements from the last two roller passes exceeded 1% of the average value of the last pass.