Abstract: In this paper, an evaluation method for defect improvement of the negative pressure forming system is proposed, which is based on CFD-DEM bidirectional coupling to simulate and analyze the negative pressure molding system under different outlet pressures, and the velocity and pressure distribution of the forming surface under negative pressure is obtained. The uniformity coefficient and the material distribution of the final screen surface are taken as important evaluation indexes to reflect the forming quality. Given the phenomenon that the velocity/pressure values are concentrated in the forming defect area, the parameters of the velocity and pressure uniformity of the forming surface are optimized to improve the forming quality. Finally, the method is applied to a negative pressure molding system for the pad of diapers. The results show that the velocity distribution of the forming surface has a significant effect on the forming defects of the pad; the forming quality can be significantly improved by setting an annular retaining ring at the outlet of the wind barn. When the thickness of the retaining ring is 140 mm, the inner diameter is 1000 mm and the relative position l=0 mm, the velocity uniformity coefficient can be reduced by 44.2% at most, and the forming quality is significantly improved by simulation and experiment.