Most surgical tools are made of metal or soft rubber, the stiffnesses of which may not be suitable for some surgeries. There is a need for stiffness-tunable materials for use in surgical tools or a mechanism for tuning the stiffness of a material without a chemical or thermal reaction. We have proposed a stiffness-tunable beam-shaped material with pneumatic access for stiffness control. We have also shown its application in grasping a soft organ stably during laparoscopic liver surgery. Its stiffness became 5.47 times greater by inducing -80 kPa negative pressure inside the device. The beam structure had a wavy composition of hard and soft rubbers. In this paper, we evaluate the finite element method (FEM) performance to determine the optimal design of the stiffness-tunable beam. FEM resulted in a 11.3-fold improvement of stiffness tuning for wavy shapes, that is, sequentially aligned pillars with an adequate pitch, and increased the stiffness, one of the shaping parameters, 11.8-fold.