Foamed concrete is a porous material widely used in buildings, but its performance is directly affected by the internal pore structure. The property of cement slurry is an important factor affecting the stability of foam. In order to explore the influence of water/cement ratio on the performance of foamed concrete, three kinds of cement slurry with different water/cement ratio (w/c) were prepared to fabricate the foamed concrete in this paper, and their dry density, compressive strength, thermal conductivity and water absorption were measured to explore the relationship between their properties and w/c. The results show that as the w/c decreases, the cement content and cement slurry viscosity increase while the density of foam concrete, concrete strength, and thermal conductivity increase gradually. However, the water absorption decreases gradually. Under an appropriate w/c, the foam concrete has the largest decrease in density compared with the pure cement slurry, reaching 41.76%. The decrease of the strength is the smallest and the compressive strength reaches 9.8 MPa. The thermal conductivity decreases the most, reaching 0.212 W/m·K, and the water absorption rate reaches 21.5%.