Multidrug resistance (MDR) is a major impediment to the success of cancer chemotherapy. One of the effective approaches to overcome MDR is to use nanoparticle-mediated the gene silence of chemotherapeutic export proteins by RNA interference to increase drug accumulation in drug resistant cancer cells. In this work, a new co-delivery system, DOX-PLGA/PEI/P-gp shRNA nanobubbles (NBs) around 327 nm, to overcome doxorubicin (DOX) resistance in MCF-7 human breast cancer was designed and developed. Positively charged polyethylenimine (PEI) were modified onto the surface of DOX-PLGA NBs through DCC/NHS crosslinking, and could efficiently condense P-gp shRNA into DOX-PLGA/PEI NBs at vector/shRNA weight ratios of 70:1 and above. An in vitro release profile demonstrated an efficient DOX release (more than 80%) from DOX-PLGA/PEI NBs at pH 4.4, suggesting a pH-responsive drug release for the multifunctionalized NBs. Cellular experimental results further showed that DOX-PLGA/PEI/P-gp shRNA NBs could facilitate cellular uptake of DOX into cells and increase the cell proliferation suppression effect of DOX against MCF-7/ADR cells (a DOX-resistant and P-glycoprotein (P-gp) over-expression cancer cell line). The IC50 of DOX-PLGA NBs against MCF-7/ADR cells was 2-fold lower than that of free DOX. The increased cellular uptake and nuclear accumulation of DOX delivered by DOX-PLGA/PEI/P-gp shRNA NBs in MCF-7/ADR cells was confirmed by fluorescence microscopy and fluorescence spectrophotometry, and might be owning to the down-regulation of P-gp and reduced the efflux of DOX. The cellular uptake mechanism of DOX-PLGA/PEI/P-gp shRNA NBs indicated that the macropinocytosis was one of the pathways for the uptake of NBs by MCF-7/ADR cells, which was also an energy-dependent process. Furthermore, the in vitro cellular ultrasound imaging suggested that the employment of the DOX-PLGA/PEI/P-gp shRNA NBs could efficiently enhance ultrasound imaging of cancer cells. These results demonstrated that the developed DOX-PLGA/PEI/P-gp shRNA NBs is a potential, safe and efficient theranotic agent for cancer therapy and diagnostics.