Tumor tissues often have a pH value lower than normal tissues, and this difference suggests a promising way of targeted cancer therapy by using pH-controlled drug delivery systems. On the basis of the mean-field theory, we present a theoretical methodology to predict the self-assembly and disassembly of pH-responsive polymers and nanoparticles in an ionic solution. It is found that vesicles, cylindrical, and spherical micelles can rapidly disassemble and release contained nanoparticles in a narrow pH range. The model is further used to study the controlled assembly of pH-sensitive drug with significantly improved encapsulation efficiency. This method is also applicable for the design of controlled delivery nanodevices for various biomedical applications.