Aiming at the problem that the rapid expansion of random tree algorithm (RRT) has no directionality and long and tortuous paths, this paper proposes an improved RRT algorithm and applies it to the path planning of manipulators. The principle of probability target bias and double sampling point selection are used to optimize the sampling points to enhance the heuristic of the algorithm; then the idea of target gravity is introduced to change the expansion direction of the new node, and on this basis, a variable step idea is proposed to enhance the effect of obstacle avoidance. The improved algorithm, RRT algorithm and the probability-biased RRT algorithm are simulated in Matlab, the results show that the execution time of the improved algorithm is 54.7% and 33.6% less than that of RRT and P-RRT in a three-dimensional environment, respectively, and the path length is also greatly improved.