The sound absorption performance of porous ceramisite is determined by its pore structure, which is mainly governed by a foaming agent and heating rate during a foaming process. By tuning the heating rate and foaming agent concentration, ceramisite with different pore structures was prepared by using flyash, cement, quick lime, and plaster as raw materials as well as ammonium acetate as a low-temperature decomposition foaming agent in this work. The phase composition, microstructure, and sound absorption performance of the prepared porous ceramisite were investigated. Results demonstrate that the apparent porosity and the pore diameter increased with the increase of foaming agent concentration, accompanied with the broadening of the pore diameter distribution. The apparent porosity is positively correlated with heating rate until the temperature is higher than 20 °C·min⁻¹, while the pore diameter is negatively correlated. The pore diameter distribution becomes narrow as a function of the heating rate. The sound absorption performance is positively correlated with the apparent porosity. An optimal pore diameter might exist, meaning diameter sizes that are larger or smaller than the optimal diameter are not conducive to the optimization of the sound absorption performance of the overall frequency band. It was determined that the curing time was not a key factor for optimizing the pore structure.