The M change characteristics in continuing current process of negative cloud-to-ground lightning discharge and artificially triggering lightning discharge are comparatively analyzed using the synchronous data of optical channel high-speed video camera pictures, electric field changes on ground and the peak very high frequency radiation. The results indicate that M process could be identified accurately from synchronous data of optical channel luminance and electric field changes on ground. There are lots of rapid changes of electric field (MP) prior to and during classic M change. And the M changes caused by lots of MP changes. There is mostly no difference in waveform characteristics between M changes and K changes and their generated physical mechanisms are similar. MP change is a rapid change of electric field corresponding to the rapid flow of charge caused by the breakdown process in the stage of continuous current process and accompanied with channel sudden brightening and high intensity radiation. Most of the M changes are unipolar, positive or negative and a few are irregular and last within 0.1 ms. Only M change in close lightning discharge exhibits a classic U-shaped structure because of its waveform mostly caused by static electric field, and its duration is about from 0.2 to 0.8 ms. MP and K changes are all caused by the breakdown. The difference between MP and K changes is that the electric charges flow into the original return stroke channel in the process of MP change, but not in the process of K change.