Abstract: In order to better realize the lightweight design of the quadcopter plant protection drone, this article focuses on the arm folding mechanism of the quadcopter plant protection drone, develops the arm folding mechanism according to the working principle, characteristics structural and loading conditions, and established a 3D model of the arm folding mechanism. Based on a static and modal analysis of the arm bending mechanism, an optimization of the topology of the unconstrained part of the arm bending mechanism is performed with the aim of optimizing the quality. Through the analysis of the optimization results, we redesigned the new arm folding mechanism and redid the static and modal analysis to ensure the safety and reliability of the arm folding mechanism. The results show that the weight of the arm folding mechanism has been reduced from the initial 231.54 grams to 204.21 grams, and the total weight of the quadcopter plant protection drone has been reduced by 109.32 grams. The strength and rigidity of the new arm folding mechanism, allows the gravity limit and the mechanism does not resonate during operation.