Abstract: The characterization of the charged particles in the thermosphere with a small ion energy analyzer was mathematically modeled, theoretically analyzed in classic electrodynamics and numerically simulated in finite element method with software COMSOL. The influence of the detection conditions, including the incident ion energy, deflection voltage, microchannel-plate (MCP) bias and environmental temperature, on the performance of the analyzer, such as the description of trajectory, transmission rate and energy resolution of incident ion, was investigated.The simulated results show that the detection conditions have a major impact. For example, a linear correlation exists between the deflection voltage and energy of incident ions; at a MCP bias of-300 V, the analyzer displays the highest energy resolution; and the temperature strongly affects the ion transmission rate but weakly influences the ion energy resolution.