The tuning of the structure of nanocarriers with fast acidic-degradation rate and high stability in physiological conditions or during storage is under intensive study. In this context, a kind of dual-pH responsive micelles with well-balanced stability, that is, fast hydrolysis in acidic environment and stability towards blood drug release at 7.4 were developed. This is achieved by the self-assembly of micelles of poly(ethylene glycol)-b-(poly ε-caprolactone-g-poly(2,2-dimethyl-1,3-dioxolane-4-yl)methylacrylate-co-2(dimethylamino)ethyl methacrylate) (mPEG-b-(PCL-g-P(DA-co-DMAEMA))) copolymers with two inert pH responsive moieties of DA and DMAEMA. The fast synergistic acid-triggered disassembly and high stability at physiological condition of the mPEG-b-(PCL-g-P(DA-co-DMAEMA)) micelles was verified by (1)H NMR, particle size and optical stability measurements, which was induced and mediated by the synergistic pH responses of the hydrolysis of the ketal in DA moieties and the switch in solubility of tertiary amino moieties (DMAEMA) under mild acid conditions. It was observed that the hydrolysis rate of the ketal could be promoted by increasing the content of DMAEMA moieties. The fast intracellular disassembly of the micelles depending on the contents of DMAEMA moieties was also traced by fluorescence resonance energy transfer (FRET). The in vitro release studies showed that the release of DOX from mPEG-b-(PCL-g-P(DA-co-DMAEMA)) micelles at mild acid condition was significantly accelerated by increasing the content of DMAEMA moieties, while greatly impeding drug release in physiological conditions. The antitumor activity of DOX-loaded micelles was studied in MCF-7 and 4T1 cells in vitro and in 4T1 tumor-bearing Balb-c mice in vivo. The results indicated the DOX-loaded micelles with higher content of DMAEMA moieties exhibited enhanced anticancer activity. Collectively, the synergistic dual-pH responsive design of mPEG-b-(PCL-g-P(DA-co-DMAEMA)) micelles provided a new route for improving anticancer drug delivery efficiency.