Nanoporous materials derived from block copolymers (BCPs), with rich morphological diversity and designable functionality, have attracted extensive interest in membrane separation, drug delivery, and catalysis. However, it is still challenging to realize the cavitation of dense BCPs through a fast and cost-effective strategy. Here, we report microwave-boosted selective swelling (MBSS), an ultrafast and universal cavitation approach, to introduce prominent porosities into BCPs within 30 s. When applying the microwave heating on swelling solvents, the dipole rotation of polar molecules leads to a drastic collision to give a quick temperature increase. Besides, the furious rotation enables a fast diffusion of swelling solvents into BCPs and accelerates the selective swelling of BCPs, achieving a pronounced volume expansion to give nanoporosities in seconds. This microwave-enabled strategy is greatly universal, capable of cavitating not only polystyrene-based BCPs but also highly rigid polysulfone-based BCPs. Interestingly, performing MBSS on BCP-coated macroporous supports realizes the ultrafast pore formation, producing composite membranes exhibiting excellent ultrafiltration performances. This microwave-based approach allows extremely facile preparation of BCP-based nanoporous materials, which is essential for their mass production and real-world applications.