We develop a new type of pH-responsive amphiphilic small molecular prodrug by conjugating folic acid with anti-tumour doxorubicin via a hydrazone bond. This prodrug is featured by high and precise drug loading (55.4wt%), which can self-assemble into micellar nanoparticles in neutral environment while disassemble in the presence of tumour cells expressing folic acid receptors or the acidic tumoral endosomal environment. The prodrug nanoparticles can effectively improve anticancer efficacy due to the features of pH-triggered drug release and targeted delivery. Moreover, in vitro cell study further indicated that the resulting prodrug nanoparticles had enhanced cytotoxicity for folic-acid-positive cells (HeLa) compared to folic-acid-negative cells (MCF-7). More importantly, the induced disassembly of prodrug nanoparticles can "switch on" the inherent fluorescence of the internalized doxorubicin drug in the tumour microenvironment, which can be used for the detection of tumour cells. We believe that this strategy can pave a new way for designing small molecular drug delivery systems and facilitate tumour diagnosis and treatment simultaneously.