A machine with variable internal stiffness can meet the performance requirement in different working environments. In the literature, linear variable-stiffness mechanisms (LVSMs) are rarer than those producing rotary motion. This paper presents an approach to design a linear variable-stiffness mechanism. The idea is to use circular slots to adjust the initial rotation of two parallel connected springs. The output force to displacement curve can exhibit zero to very large stiffness depending on the rotation of the springs. The merit of the proposed LVSM is that very small and very large stiffness can be simultaneously achieved in a compact space. To further reduce size and complexity, semi-circular planar springs are proposed to replace commercially available coil springs. Force and stiffness analyses are presented to design a LVSM with the largest stiffness variation. The effects of various parameters on the stiffness variation are discussed. The results are numerically verified with a prototype illustrated.