Abstract: Linear/nonlinear switching extended state observer (SESO) is a newly proposed state error-based switching observer that switches between the linear extended state observer (LESO) and the nonlinear extended state observer (NLESO) when performing state estimation in order to realize high control precision and strong anti-disturbance ability. Presently, there are no studies on the convergence and stability analysis of this observer in the frequency domain. Considering this issue, the structure and parameter tuning of SESO is introduced for a second-order plant; then the convergence and stability of the observer are analyzed in the frequency domain using the equivalent gain method, and the function is described. The results show that the third-order SESO has good convergence and can estimate the states and "total disturbance" of the system, and that if the parameters are tuned properly, limit cycles will not exist.