Compared to manual therapy, robot therapy can provide more intensive and accurate rehabilitation training. However, most current devices are built on bulky and complex multi-degree-of-freedom (multi-DOF) mechanisms. Recently, 1-DOF mechanisms have gained popularity due to their portability and simplicity. Existing synthesis methods for 1-DOF mechanisms focus primarily on computing the optimal mechanism dimensions such that kinematic error between the task and the mechanism output is minimized. In cases where the kinematically feasible solutions become impractical under engineering circumstances, designers may need a handle to intervene in the synthesis process; moreover, the force interactions between the mechanism and users should also be considered to encourage the active participation of users for effective physical recovery. In this paper, we combine kinematic and kinetostatic synthesis to develop an interactive rehabilitation mechanism design system, taking into account task specifications on rehabilitation motion and gravity balancing. To enable interactive design, users are invited to manage the task movement via kinematic tolerance-oriented variation, thus providing the flexibility to address practical constraints. To compensate the gravity, torsional springs are attached to the actuated joints of the mechanism and human limb, and designed based on the principle of static balancing. For presenting a systematic, general, and defect-free design methodology for 1-DOF rehabilitation mechanisms, the synthesis model is formulated in a Fourier way to better accommodate different mechanism types and continuous limb motion. Examples of the upper- and lower-limb rehabilitation mechanism design are given in the end to demonstrate the validity of the proposed method.