Aiming at the joint robot path plan in unknown environment, the paper adopts the method of obstacle avoidance in X-Y plane. The obstacles exist in the Cartesian space are transformed into the joint blind regions in the Joint (C) space through geometry principle and inverse kinematics. The simulation using the partial particle swarm optimization (PSO) algorithm is utilized in seeking the angles that can avoid obstacles. Finally the path in the Cartesian space is obtained through transforming angles. The method is verified to be simple and effective.