Natural scoria has issues regarding field compaction and testing indices, which needs to be resolved. Laboratory soil testing was conducted on natural scoria samples collected in field to reveal its vesicular structure, mineral phase, mechanical and physical properties. The impact of silty sand addition and mixing ratio on the engineering properties of mixed soils was explored by field compaction tests. Results indicate that the particles of scoria have rough surface and extensive inner voids, the crushing ratio is positively related to the open void ratio, and their L.A. abrasion loss value was less than 50.0%; the presence of fine aggregates can mitigate grain crushing. The modulus of subgrade reaction (K₃₀) for stabilized fill first increases then decreases with the rising of mixing ratio, which coincides with the transition mode of three soil structures, namely, fine-graded, stone matrix, and open-graded types. The stabilized soils with volumetric mixing ratio of 4∶1~2∶1 can be classified as a stone matrix type, with specimen at mixing ratio of 2∶1 achieving the best performance in practice. In summary, the particles of scoria are found to have desirable strength against grain crushing, and the performance of scoria–silty sand mixtures are much better than natural scoria; inter-particle void ratio is suggested to be taken as an indicator of quality assessment for subgrade compaction.