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In this study, pulsating hydroforming of tube in a box die is simulated using the three dimensional finite element method to investigate the mechanism of improvement of the corner filling. In addition, the results obtained from the simulation are compared with experimental observations, and the validity of the simulation results is verified. Based on a better understanding of the mechanism of improvement of the corner filling, a new pulsating pressure path is proposed to increase the corner filling. It has been shown that the proposed pulsating pressure path is more effective in increasing the corner filling of the box shape tube hydroforming process.