Graphene oxide (GO) is an important two-dimensional material since it is water soluble and can be functionalized to adapt to different applications. However, the current covalent functionalization methods usually require hash conditions, long duration and sometimes even multi-steps, while noncovalent functionalization is inevitably unstable, especially under physiological environment where competing species exist. Diselenide bond is a dynamic covalent bond and can respond to both redox condition and visible light irradiation in a sensitive manner. Thus, in this work by combining the stimuli response of diselenide bond and the oxidative/radical attackable nature of GO, we achieved the in situ covalent functionalization of GO simply by stirring GO with diselenide-containing molecules in aqueous solution. The covalent functionalization was proved by FT-IR, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Atomic Force Microscopy (AFM), Thermogravimetric Analysis (TGA), etc. and the functionalization mechanism was deduced to involve both redox reaction and radical addition reaction according to the XPS, Atomic Emission Spectroscopy (AES) and Raman spectroscopy. Moreover, we modified GO with a biocompatible diselenide-containing polymer (mPEGSe)2 and found selenium-functionalized GO could modulate the balance of reactive oxygen species (ROS). GOSe could decrease ROS level by accelerating the reduction of peroxides when the ROS concentration is high while boosting the ROS level by in situ generating ROS when its concentration is relatively low.