Abstract: To address safety concerns arising from restricted worker movement due to sub-optimal robotic arm trajectory planning in a human-robot collaborative assembly workspace, a trajectory optimization method for robotic arms based on the principle of minimum coverage area was proposed. Firstly, the coverage area of a robotic arm is defined. The problem is formally described. An optimal trajectory model is then constructed using a 3-5-3 sectional polynomial interpolation, with the objective of minimizing the total coverage area. Secondly, an improved bat algorithm is introduced. It reveals the novel procedures for population initialization, velocity update, multi-structure local search, and parameter adaptation. Finally, case studies and comparative analyses are given. They demonstrate that the proposed method achieves strong global search capability and fast convergence, enabling efficient computation of safe and optimal robotic arm trajectories for human-robot collaborative assembly.