In the field of building waterproofing, "rigidity" and "flexibility" have never been opposing choices but rather complementary solutions. High-performance waterproofing coatings represented by polyurethane (MPU), with their exceptional elasticity, seamless formation, and perfect integration with substrates, are becoming indispensable "flexible defenses" in modern waterproofing engineering. When they work in synergy with "rigid waterproofing" systems such as cementitious capillary crystalline materials, they form a perfect "rigid-flexible, complementary" waterproofing barrier. This article focuses on the unique advantages of polyurethane waterproofing coatings, exploring their irreplaceable role in complex details, bathrooms, basements, and other scenarios, and analyzes the development trends of new solvent-free and spray-applied coatings.