Silty soil is widely distributed in some areas of China. Due to its low cohesion and poor water retention, if it is used as the subgrade soil, the compaction degree as the compaction control index, after the subgrade forming, it is prone to collapse in the rainy season. In order to ensure the strength and water stability of the silty soil subgrade, it is proposed to use the air void ratio as the silty soil subgrade compaction control index. This research takes the widely distributed low liquid limit silty sand soil in Shanxi Province as the research object. CBR test, resilience modulus test, and direct shear test were carried out. Through laboratory tests and field verification, it is concluded that air void ratio is more reasonable as a control index for silty soil subgrade compaction, and 12% air void ratio can be used as the compaction control index in this area. It not only ensures that the compacted soil has a large dry density, but also can control the water content of the soil.