The post-arc residual plasma of the vacuum arcs in the breaking process is an important parameter, which is used to indicate the breaking capacity of the vacuum circuit breakers. Based on the basic principle of the electron saturation region for the probe diagnostics, the paper proposes a measuring method of electron density in residual plasma. The structure and principle of the method are analyzed. The probe structure and control system of the probe diagnostics are designed. The electron density of the residual plasma is measured by the single probe based on the dismountable vacuum chamber. The vacuum arcs development is observed by the high speed camera. The influence of the current magnitude and contact structure on the decay process of the residual plasma is investigated. Finally, the validity of the measurement method is verified by comparing with other diagnostic methods, which provides a low-cost and effective diagnostic method for future research on the micro-characteristics of vacuum circuit breakers.