The work aims to design the structure of the bottom forming mechanism of gable-box filling machine with respect to its unreasonable design and unstable operating, etc. In combination with its technological requirements, the bottom forming mechanism of gable-box filling machine was designed. By analyzing the design of the bottom forming mechanism, the motion displacement of the center rod, the opening angle range of the swing rod and the relationship between the driving force and the carton reverse thrust were calculated. A 3-D model was built based on SolidWorks and imported into ADAMS to simulate the action process of the forming mechanism. The simulation analysis results showed that the motion track of the bottom forming mechanism of gable-box filling machine was reasonable, and the opening angle of swing rod was within the theoretical calculation range. In such case, the bottom forming could be completed during one-time motion cycle. The design not only saves space and reduces costs, but also improves the bottom sealing efficiency of the existing gable-box filling machine, which has a high application value.