An improved-RRT-algorithm-based motion planning method for a mechanical arm grabbing dispersedly stacked pistons comprises the steps that the pose of the six-axes mechanical arm is described with a D-H method first, and then a D-H model of the six-axes mechanical arm is obtained through calculation; then an ROS simulation system of a piston workblank feed platform is built; and finally motion planning of the mechanical arm is completed mainly with an improved RRT algorithm. The planning process is divided into two stages by the improved RRT algorithm, in the first stage, an obstacle avoidanceproblem caused by the obstacle pistons is solved, the planning speed of the algorithm is increased, the first stage is divided into three steps, and 3D operation is conducted on the area around the target piston in the first step; and in the second stage, a transient test is introduced in, a control function is refined, the shortest path principle is constructed, and thus the quality of the path planned with the improved RRT algorithm is improved. The motion planning method is suitable for the mechanical arm grabbing the dispersedly stacked pistons, and planning time is saved.