The preparation of high-expansion open-cell foam for oil spill clean-ups is important, but still challenging with linear isotactic polypropylene (PP). Therefore, a cooling batch foaming method was designed to fabricate the high-expansion PP foams using supercritical CO2 as a blowing agent. To investigate the relation between the crystallization and foaming of PP, an in-situ visualization system was employed. It is found that the CO2 dissolved in polymer melt depresses the crystallization temperature and nucleation of PP. When the foaming is triggered before the crystallization, high-expansion foams can be prepared. Moreover, foaming occurring before crystallization helps to produce an open-cell structure owing to a structural inhomogeneity induced by the PP crystallization. According to the hydrophobicity and oil absorption capacity tests, the open-cell foam prepared at 20 MPa and 135 °C exhibits a large water contact angle of 151.5° and a high adsorption capacity of 48.9 g/g for carbon tetrachloride. Further, it exhibits an excellent reusability for oil recovery in the cyclic adsorption and squeezing process. Therefore, the fabricated high-expansion open-cell foam shows good application prospects in oil spill clean-up fields.