As a cross-linked three-dimensional network of hydrophilic polymer materials, hydrogels have similar characteristics to biological tissues and can absorb a large amount of water. As a kind of smart hydrogel, pH-sensitive hydrogel has a certain amount of pH sensitivity due to its structure containing a large number of basic or acidic groups. With these properties, such hydrogels have attracted much attention of researchers in many research fields such as biology, medicine, physics, environment and textiles in recent years. This review mainly focuses on three aspects: the response mechanism, classification and application of pH-sensitive hydrogels. Firstly, the response mechanism of pH sensitivity is reviewed from three aspects: response process, influencing factors and swelling-diffusion model. According to the pH sensitivity of such hydrogel, it can be divided into two types: swelling-shrinking type and sol-gel type. Further, the swelling-shrinking type can be divided into three types: anionic, cationic and amphoteric ionic according to the pH working range of such hydrogel, and the sol-gel type can be divided into borates, hydrazine and imines. In the field of applied research, this article summarizes the research status of such hydrogels in some popular fields such as medicine, environment, biology, intelligent monitoring and functional materials. Finally, the future of such hydrogel is prospected.