The invention discloses a deformable bottom die for pressing back panels, comprising a die set (1); wherein, the die set (1) is provided with a main die (2) suitable for the size of the back panel; split dies (3) are respectively arranged at both sides of the main die (2) of the die set (1); the split dies (3) can be combined with or separated from the main die (2) to form die surfaces suitable for the back panels of the clothes in different sizes; the split dies of the invention can translate toward the inside or outside so as to combine the main die to form the die surfaces suitable for theback panels of the clothes in different sizes to be suitable for the shape changes of different sizes of the back panels of shirts. When the clothes in different sizes and of different styles are pressed, preset size or style patterns are chosen on a control box; driven by the motion of electrical components, the die can automatically change to corresponding shape sizes, thus being convenient andfast in operation. Therefore, the cost and time in changing dies are greatly saved and the machine use ratio is improved.