In order to improve the harvesting efficiency of forest-fruit, the structure properties of the harvesting robot arm was introduced. Based on the modal analysis theory, the harvesting robot should have more reasonable structure and dynamic performance, and a finite element model of the harvesting robot arm was established by utilizing the software of ANSYS Workbench. Three kinds of typical pose were selected, further more, the preceding six steps natural frequency and mode were obtained. Finally, the weak parts of the robot arm were found out and relative improving suggestion was put forward, which laid foundation for the optimized design.