Abstract: The constraint of the multi-needle surgical planning of the robot-assisted liver cancer coagulation therapy system for the large liver cancer therapy is that the path of the needle should avoid the ribs and blood vessels and other tissues in the abdominal cavity and also should avoid interfering with the inserted needle. Based on the single needle operation process of liver and kidney cyst puncture, a multi-needle surgical process is designed and the necessary functions for the medical robot in large tumor therapy are listed. And based on the boundary of the needle's collision-free reachable workspace, the core algorithm and the optimization criterion of multi-needle surgical planning are proposed. In the simulation experiments, the boundaries of needle's collision-free reachable workspace are successfully solved, when there are ribs, blood vessels, and inserted needles in celiac, and a multi-needle surgical strategy is achieved, which ensures the needle insertion path does not interfere with obstacles, and minimizes the injure to the patient.