Robots can protect healthcare workers from being infected by the COVID-19 and play a role in throat swab sampling operation. A critical requirement in this process is to maintain a constant force on the tissue for ensuring a safe and good sampling. In this study, we present the design of a disposable mechanism with two non-linear springs to achieve a 0.6 N constant force within a 20 mm displacement. The nonlinear spring is designed through optimization based on Finite Element Simulation and Genetic Algorithm. Prototype of the mechanism is made and tested. The experimental results show that the mechanism can provide 0.67±0.04 N and 0.57±0.02 N during its compression and return process. The proposed design can be extended to different scales and used in a variety of scenario where safe interacting with human is required.