Recently, block copolymer micelles have attracted widespread attention due to their controlled biodegradability and excellent loading capability. Unfortunately, the poor in vivo stability and low delivery efficiency of drug-loaded micelles greatly hampered their biomedical applications. Herein, we develop a new kind of biodegradable magnetite/doxorubicin (Fe3O4/DOX) co-loaded PEGylated organosilica micelles (designated as FDPOMs) with both high circulating stability and smart GSH-triggered biodegradability for magnetically targeted magnetic resonance imaging (MRI) and tumor chemotherapy. The FDPOMs are prepared by the self-assembly of biodegradable polycaprolactone-block-poly(glutamic acid) (PCL-b-PGA), a chemotherapeutic DOX drug and Fe3O4 nanoparticles in an oil/water system, subsequent organosilica cross-linking with 3-mercaptopropyltrimethoxysilane (MPTMS) molecules and surface PEGylation. The resultant FDPOMs exhibit excellent dispersity and stability in biological media, remarkable T2-weighted MR imaging capability, unique GSH-responsive release behavior and selective toxicity to tumor cells. The in vivo experiments show that the FDPOMs not only have improved MR tumor imaging capability, but also exhibit high anti-tumor efficacy due to the strong magnetic targeting ability under an external magnetic field. Consequently, the FDPOMs are promising candidates for magnetically targeted MR imaging and imaging-guided tumor chemotherapy.