Vacuum circuit breaker has many advantages

such as compact structure

convenient maintenance

no greenhouse gas emission

and strong interruption ability. Therefore

it is widely used in medium-voltage power grid.Vacuum interrupter is the core component of vacuum breaker

and its internal pressure（vacuum degree）is the key factor to reliably turn on and off the circuit. However

vacuum degree online detectionis still an international problem

and there is no effective vacuum degree online detection method for vacuum interrupter. This paper study the evolution of plasma under different pressures and different laser energies by plasma imaging. The experimental results show that the environmental pressure has a significant influence on the expansion dynamics of plasma

and there are obvious interval effect. At the same time

the evolution of plasma under different laser energy is similar.In order to quantify the change of plasma

this paper proposes three characteristic parameters of plasma: the distance from the plume barycenter to the target surface

the integral of plasma intensity

and the plume length of plasma. The experimental results show that the above three parameters can commendably extract the plasma characteristics at different vacuum degrees.This work can lay the foundation for the practical application of vacuum degree detection method based on laser-induced plasma technology.