Lymphadenectomy generally accompanies various oncology surgeries to remove infected cancer cells. However, there are two limitations in robot-assisted lymphadenectomy: 1) lymph nodes are not visible during operation since they are hidden by the superficial fat layer; 2) intra-operative bleeding may occur during lymph node removal caused by collisions between surgical instruments and delicate blood vessels (arteries or veins) near the lymph nodes. Therefore, we propose a framework integrating augmented reality and virtual fixtures to address these limitations. Augmented reality intra-operatively visualizes the hidden lymph nodes by projecting the corresponding 3D pre-operative model, and virtual fixtures are used to provide force feedback to surgeons to avoid possible collisions when they operate the surgical instruments to resect the lymph nodes surrounding the blood vessel. Ten human subjects were invited to perform an emulated lymphadenectomy based on the da Vinci robot in a dry lab. Experimental results demonstrated that the proposed framework can keep localizing the hidden lymph nodes, and reduce the number of collisions (21% and 48% reduction rates using two different force models compared to the standard setup, respectively) between the instruments and the delicate blood vessel during lymph node resection. It shows the potential to enhance the safety of robot-assisted lymphadenectomy.