To improve the performance of a force interaction system, it is necessary to understand the effect of respective parameter on the system through modeling and analysis. Considering a 1\|DOF force interaction system, the human operator in the system was modeled as a combination of passive impedance and an active input, and the virtual environment was assumed to be passive. Through analogous treatment between the force interaction system and an electrical network system, two-port stable theory was used to derive the stability condition of the system. The effect of transmission stiffness on the system stability was particularly emphasized. The results provide important insight to the system performance and are applicable to the design of a force interaction systems.