Foam concrete, as an important construction and building material, mainly consists of small inner pores (produced by preformed foam) and foam walls (i.e., the concrete surrounding the small inner pores). The effect of density and air volume quantity on compressive strength has been investigated in many previous studies. However, the findings on the relationship between compressive strength and water-to-cement ratio (Rwc) are controversial from different studies. The possible reason may be the effect of sand-to-cement ratio (Rsc), which has not been considered in pervious studies. In this study, a series of compressive tests on foam concrete with various Rwc and Rsc were conducted at a fixed air volume quantity. The results show that when Rwc was 0.5–1.0, the compressive strength increased along Rwc, different from the change of the concrete without foam. The enhance effect from the foam walls was dominant. When Rwc was larger than 1, the slurry was too thin to preserve the bubble for the Rsc of 2. However, for the Rsc of 5, the slurry performed well and its compressive strength remained constant, which was different from the increase stage with Rwc of 0.5–1.0. It was because of the enhanced effect caused by the decrease in the number of small holes, which almost offset the weakening effect for the Rwc on the strength. The enhance effect due to the decrease in the number of small holes can be normalized by the water-to-solid ratio (Rws). Except the results in the constant stage, the compressive strength increased with the increase of Rws, irrelevant to the Rsc. It indicates that the sand and cement had the same function on the decrease in the number of small holes. In order to get the same compressive strength, the cement can be replaced by the sand in the increase stage. The research results are expected to improve the quality control and the engineering efficiency of foam concrete.