The cement-mixed soils technique is an efficient solution to improve the ground geomechanical properties in infrastructures and construction projects. This paper presents a systematic laboratory study to investigate the optimum water-to-cement ratio existence on the unconfined compressive strength. The soil compaction is controlled during these tests. The results showed that the maximum unconfined compressive strength was not only controlled by the porosity but also the water-to-cement ratio. The results indicated that the optimum range of water-to-cement ratio to mobilize the maximum strength is between 0.75 and 1.25 while the optimum water content is around 15%, and is quite stable for various cement content. Besides, the unconfined compressive strength decreased with increasing the water-to-cement ratio, and the reduction is more significant for higher cement contents. It seems that high cement content could be not sufficient for a large water-to-cement ratio. Finally, a new index, namely, the combined volume ratio is proposed for the strength analysis of cement-mixed soils. Using this index, the unconfined compressive strength of cement-mixed soils can be reasonably predicted. The combined volume ratio allows selecting the ratio of water and cement volume to ensure that the void volume is minimized, and the strength is therefore maximized.