The drilling and sampling method is commonly used to inspect the construction quality of cast-in-place foamed cement paste blocks; however, it results in the destruction of the blocks. The air-coupled impact-echo method, a non-contact and non-destructive technique, offers high efficiency for material inspection. This study developed an impact device to investigate the air-coupled impact-echo spectrograms of foamed cement paste blocks with the densities ranging from 0.5 to 1.5 g/cm3. For adjusting the block’s thickness vibration mode in testing, the shape factor was introduced. The effects of specimen density, impact ball diameter, and impact location on the shape factor were investigated. The results showed that the shape factor decreased as both the specimen density and impact ball diameter increased, attributed to wave reflection and diffraction in voids. The impact location significantly affected the dominant frequency owing to the geometrical boundary effect, and a threshold (0.03 of the ratio DsDlDm/H3) was proposed to assess boundary effects. This study verifies the feasibility of air-coupled impact-echo testing for foamed cement paste blocks and introduces novel parameters to characterise boundary effects. These findings provide valuable strategies for applying non-destructive testing to foamed cement paste structures in engineering practice.