For a certain cartridge case, aiming at the problem of micro-folding in the side wall of bottom fire chamber after the bottom forming of cartridge case in the initial design scheme, the micro-folding occurred during the indenting was determined by microstructure observation and numerical simulation, and the causes of micro-folding were clarified. During the indenting process, the radial and axial flow of metal at the partition was significantly faster than that in the inner wall, resulting in a continuous reduction of curvature radius for the fillet at the connection zone between partition and inner wall, until there was a micro-folding defect. At the same time, the micro-folding defect retained and moved down a short distance with the axial flow of metal in the side wall during the heading process, which was consistent with the actual observed position of micro-folding defect. In order to improve the forming quality of cartridge case and avoid the defects, an improved scheme of modifying the dimension of indenting die to improve the metal flow during the indenting process was proposed. The numerical simulation and process test results show that by the improved scheme, there is no micro-folding defect in the indenting and heading cases, and other dimensions meet the design requirements. Thus, the improved scheme can meet the actual production requirements and improve the forming quality of the bottom for the cartridge case.