The die-closing issue of hydroforming process for large size circular components was researched, and the comparing analysis was conducted between press machine aided mechanical die-closing method and bolt aided mechanical die-closing method by numerical simulation. Due to the large size of component, it was difficult to choose a suitable press for closing force and work table area. The simulated results show that the dimensional accuracy of formed component cannot be guaranteed when the work table size of press machine is smaller than the die projection area, even though the closing force is sufficient. However, by the bolt aided mechanical die-closing method, the homogeneous closing force on the die can be supplied by optimizating the number and diameter of bolts. Then, the hydroforming die based on bolt-aided mechanical die-closing method was developed, and the tests were carried out. The experimental results indicate that a good effect can be obtained for large-size component during hydroforming process by the bolt-aided mechanical die-closing method. The final dimensional accuracy of formed component can meet the design requirement.