A series of microporous open-cell poly(vinyl formal) (PVF) foams were obtained by cross-linking poly(vinyl alcohol) (PVA) with different contents of formaldehyde during the first acetalization process. In this reaction, water acted not only as the solvent of PVA but also as the pore-forming agent, which made this method more concise and environment-friendly. However, the PVF foam walls were not strong enough to maintain the initial pore shapes and sizes. The second acetalization process was used to increase the acetalization degree to enhance the strength of the foam walls. The poly(vinyl formal) foams obtained by the two-step acetalization method (TPVF) were able to keep the pretty microporous structures during the drying process. The materials with highly microporous structures exhibited a good ability to absorb normally incident sound. Therefore, the microporous open-cell TPVF foams have great potential in the application of sound absorption materials.