As an important physical parameter, the electron number density is usually used to analyze the change process of the interaction between high power microwave and plasma. The change characteristics of electrons in plasma was studied by combining the wave equation, the electron transport equation and the heavy species transport equation by using the fluid approximate method, and the average electron number density and electron energy in the plasma region were calculated with the help of the software COMSOL. Result shows that the electron avalanche effect in the plasma appears as HPM(high power microwave) propagates inside the plasma, the electron numerical density increases rapidly, and the peak value shift towards the incident direction of HPM. In addition, it can be admitted that the electron energy increases with the HPM and decreases as the electron density increases.