<p>Based on several nonlinear and linear stiffness modeling methods, nonlinear stiffness characteristics of a collaborative robot (cobot) are analyzed in this paper. The analyzed characteristics include not only the linear stiffness characteristics of robots, but also the nonlinear stiffness performance of joint transmission systems, the change rate of Jacobian matrix under loads and the changes of poses caused by the load and deadweight. Firstly, a nonlinear stiffness modeling method based on the large deformation assumption is built, which is combine the finite element substructure method (FESM) with the virtual joint method (VJM). Then, a linear stiffness model, a stiffness model considering nonlinear stiffness characteristics of transmission systems, the enhanced stiffness model considering nonlinear stiffness characteristics of transmission systems are established respectively. Finally, the influence of nonlinear factors on robot stiffness characteristics are analyzed in specified poses and workspace, and influence level of nonlinear factors are clearly defined.</p>