Abstract: The residual plasma electron density is an important parameter to characterize the post-arc microcosmic characteristics of the vacuum circuit breaker. In this paper, the working principle of the probe diagnosis method is analyzed, and a probe diagnosis method based on the electron saturation region of vacuum arc residual plasma is proposed. The probe structure, probe control unit and data processing system are designed, and the diagnosis system of residual post-arc plasma of the vacuum circuit breaker based on a single probe is developed. A diagnostic experiment platform was built by using a detachable vacuum arc extinguishing chamber, and the influence of breaking current, contact structure and measuring position on the spatial distribution of post-arc electron density was systematically studied. Research shows that: the initial electron density measured by the post-arc plasma probe diagnostic system of the vacuum circuit breaker is in the range of 10¹⁰−10¹¹ cm⁻³, and the decay time is 30−40 μs. The variation law is consistent with the back-arc sheath theory, and the comparison with the measurement results of other diagnostic methods at home and abroad verifies the feasibility and effectiveness of the probe diagnosis method. It provides an effective and simple method for diagnosing electron density after the vacuum arc.