Compared with extrusion foaming, semi-solid-state batch foaming with CO2 can produce foams with small bubble size and high expansion ratio using the polymers with regular melt strength as the raw material. However, due to the presence of crystal regions, semi-solid-state batch foaming of polypropylene (PP) is a challenge. In situ melting and rheological behaviors of PP were measured using high-pressure differential scanning calorimetry and high-pressure rheometer, respectively. Through the CO2 pressure swing saturation strategy, the PP matrix can dissolve more CO2 in a shorter time, thereby effectively widening the foaming temperature range, increasing the expansion ratio, and shortening the saturation time. The Gotsis' model was used to correlate the high-pressure rheological data to obtain the characteristic relaxation time and rigid modulus of PP, and then, the bubble growth stability during the foaming process was simulated. According to the Considere criterion, unstable growth factors were introduced to judge the quality of the foaming results. It was found that when the unstable growth factor was higher than 0.3, the bubbles began to collapse and rupture.