Biodegradability of inorganic nanoparticles is one of the most critical issues in their further clinical translations. In this work, a novel "metal ion-doping" approach has been developed to endow inorganic mesoporous silica-based nanoparticles with tumor-sensitive biodegradation and theranostic functions, simply by topological transformation of mesoporous silica to metal-doped composite nanoformulations. "Manganese extraction" sensitive to tumor microenvironment was enabled in manganese-doped hollow mesoporous silica nanoparticles (designated as Mn-HMSNs) to fast promote the disintegration and biodegradation of Mn-HMSNs, further accelerating the breakage of Si-O-Si bonds within the framework. The fast biodegradation of Mn-HMSNs sensitive to mild acidic and reducing microenvironment of tumor resulted in much accelerated anticancer drug releasing and enhanced T1-weighted magnetic resonance imaging of tumor. A high tumor-inhibition effect was simultaneously achieved by anticancer drug delivery mediated by PEGylated Mn-HMSNs, and the high biocompatibility of composite nanosystems was systematically demonstrated in vivo. This is the first demonstration of biodegradable inorganic mesoporous nanosystems with specific biodegradation behavior sensitive to tumor microenvironment, which also provides a feasible approach to realize the on-demand biodegradation of inorganic nanomaterials simply by "metal ion-doping" strategy, paving the way to solve the critical low-biodegradation issue of inorganic drug carriers.