Injectable hydrogels have been widely investigated for applications in biomedical fields, for instance, as biomimetic scaffolds mimicking the extracellular matrix (ECM). In addition to as scaffolds for mechanical support and transferring of nutrients, the dynamic bioactivity of ECM is another critical factor that affects cell behavior. In this work, a novel injectable poly(L-glutamic acid)-based hydrogel decorated with RGD was fabricated. The presentation of RGD significantly enhanced the cell-matrix interaction and promoted cell adhesion and proliferation. Moreover, the cell-adhesive RGD was conjugated to the network via a disulfide bond, so that the density of RGD and the bioactivity of hydrogel can be well controlled by tuning the RGD content through treating with glutathione. As a result, the cell behaviors on the hydrogel can be tuned on demand. The injectable hydrogel with controllable bioactivity may provide an interesting strategy to develop a scaffold mimicking ECM that can regulate cell adhesion dynamically.