The　development　of　stimuli-responsive　theranostic　platforms　is　of　great　demand　for　efficient　cancer　treatment　because　they　can　enhance　diagnostic　specificity　and　sensitivity.　In　this　work,　we　report　a　pH-responsive　theranostic　nanoplatform　based　on　FeOOH　clusters　loaded　mesoporous　silica　nanoparticles　(Fe@MSNs).　The　as-synthesized　Fe@MSNs　possess　activatable　T1　magnetic　resonance　imaging　(MRI)　performance　that　can　respond　to　the　acidic　microenvironment　of　solid　tumor　to　turn　on　T1　singals　by　releasing　paramagnetic　Fe3+　ions.　The　Fe@MSNs　are　biocompatible　without　appreciable　cytotoxicity.　Moreover,　the　unique　mesoporous　structure　endows　the　Fe@MSNs　with　significant　advantages　to　effectively　deliver　chemotherapeutic　drug　for　inhibiting　the　growth　of　solid　tumor.　We　believe　that　this　novel　pH-responsive　theranostic　nanoplatform　holds　great　promise　in　cancer　treatment.　©　2018,　Science　China　Press　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.