A borated aminated cellulose citrate (BACC) inhibitor was synthesized from bagasse cellulose with citric acid, diethanolamine, and boric acid by mechanical activation-assisted solid-phase reaction (MASPR) technology. The BACC was characterized using FTIR, XPS, and solid-state CP/MAS NMR. The effect of the BACC inhibitor on the corrosion inhibition of A3 steel in simulated cooling water was investigated using weight loss and electrochemical measurements. It was found that BACC mainly acted as a mixed-type inhibitor in simulated cooling water and predominantly controlled the anodic reaction; furthermore, the inhibition efficiency increased with increased BACC concentration. BACC also showed excellent scale inhibition in simulated cooling water. XRD and SEM analyses of the scale deposits showed that the surface morphology and size of the scale deposits were changed by BACC. These results indicated the potential of BACC as an effective corrosion and scale inhibitor in cooling water.