Aiming at the problems of complicate process for robotic arm assembly and difficult analysis for line balancing, a timed Petri network modeling method based on topdown/downtop was proposed. According to the actual assembly characteristics of the robotic arm, the definition rules and construction methods of the assembly line model were studied. After introducing the model hypothesis, a timed Petri net model based on the process was built with the idea of topdown. Then a timed Petri net model based on any procedure was built with the idea of downup. Then the models above were combined to establish the timed Petri net model of the entire assembly line. Finally, the feasibility of the modeling method was verified by establishing and analyzing the reachable tree of the corresponding model.And this study has great application value in guiding the construction of robot arm assembly line.