Supramolecular self-assembly has gained increasing attention to construct multicomponent drug delivery systems for cancer diagnosis and therapy. Despite that these self-assembled nanosystems present surprising properties beyond that of each subcomponent, the spontaneous nature of co-self-assembly causes significant difficulties in control of the synthesis process and consequently leads to unsatisfactory influences in downstream applications. Hence, we utlized an in situ dynamic covalent reaction based on thiol-disulfide exchange to slowly produce disulfide macrocycles, which subsequently triggered the co-self-assembly of an anticancer drug (doxorubicin, DOX) and a magnetic resonance imaging (MRI) contrast agent of ultrasmall iron oxide nanoparticles (IO NPs). It showed concentration regulation of macrocyclic disulfides, DOX, and IO NPs by a dynamic covalent self-assembly (DCS) strategy, resulting in a stable codelivery nanosystem with high drug loading efficiency of 37.36%. More importantly, disulfide macrocycles in the codelivery system could be reduced and broken by glutathione (GSH) in tumor cells, thus leading to disassembly of nanostructures and intellgent release of drugs. These stimuli-responsive performances have been investigated via morphologies and molecular structures, revealing greatly enhanced dual-modal MRI abilities and smart drug release under the trigger of GSH. Moreover, the codelivery system conjugated with a targeting molecule of cyclic Arg-Gly-Asp (cRGD) exhibited significant biocompatibility, MR imaging, and chemotherapeutic anticancer effect in vitro and in vivo. These results indicated that in situ dynamic covalent chemistry enhanced the control over co-self-assembly and paved the way to develop more potential drug delivery systems.