We report a process for the functionalization of graphene prepared by chemical vapor deposition with plasma treatment. A pH sensor is prepared by the treated graphene. The micro-structure of graphene before and after plasma treatment is investigated by contact angle meter, Raman spectrometer, X-ray photoelectron spectroscope and atomic force microscope. With plasma treatment time increased, the contact angle with water droplet is gradually reduced. Raman spectroscope result indicates the D and D′ defect peaks start to merge and 2D peak is suppressed and broadened. In the X-ray photoelectron spectroscopy, plasma treatment leads to the oxygenated functional groups on the graphene surface. It is found that surface roughness of graphene is increased after plasma treatment. Under the optimized conditions, the sensitivity of graphene pH sensor with plasma treatment can be improved by 3 times.