The study investigated the changes in random copolymerized polypropylene pipe (PPR pipe) after contacting with polyurethane waterproof coatings and xylene diluents under different conditions. The tensile properties and thermal properties of PPR pipe were measured, and the impacts of polyurethane waterproof coatings and xylene diluents on the properties of PPR pipe were discussed. The results show that small organic molecules of polyurethane waterproof coatings and xylene diluents penetrate into the PPR pipes, reducing the yield strength and elongation at break of the PPR pipes. Small organic molecules from polyurethane waterproof coatings penetrate into the gaps between the molecular chain of the PPR pipe, increasing the volume between the molecular chain segments. The xylene diluent accelerates the penetration of small organic molecules from polyurethane waterproof coatings. The swelling of the PPR pipe is more obvious, and the loss of mechanical strength increases. The small organic molecules have an adverse effect on the oxidation resistance of PPR pipe. When the polyurethane waterproof coatings directly contact PPR, the small organic molecules from the polyurethane waterproof coatings will penetrate into the PPR pipe, affecting the mechanical properties and oxidation resistance of the PPR pipes.