Retinal vein cannulation (RVC) is a potential treatment for retinal vein occlusion (RVO). Manual surgery has limitations in RVC due to extremely small vessels and instruments involved, as well as the presence of physiological hand tremor. Robot-assisted retinal surgery may be a better approach to smooth and accurate instrument manipulation during this procedure. Motion of the retina and cornea related to heartbeat may be associated with unexpected forces between the tool and eyeball. In this paper, we propose a force-based control strategy to automatically compensate for the movement of the retina maintaining the tip force and sclera force in a predetermined small range. A dual force-sensing tool is used to monitor the tip force, sclera force and tool insertion depth, which will be used to derive a desired joint velocity for the robot via a modified admittance controller. Then the tool is manipulated to compensate for the movement of the retina as well as reduce the tip force and sclera force. Quantitative experiments are conducted to verify the efficacy of the control strategy and a user study is also conducted by a retinal surgeon to demonstrate the advantages of our automatic compensation approach.