I tried to increase the height of the beam, but every time I increased beam height, it was far more than I need. Finally I found out the difference of the beam height and the height of the cavity is very tiny, before the beam is lower is because a pretty slow reduction of the beam height. So I adjusted the beam path again and made the beam goes into the cavity from the center in vertical way, but this time the situation went back to before, there were two beams in the horizontal direction, but it seems better since there are not very strange behavior of the two beams, and I can see the mode is exactly some higher mode. The alignment will kept doing tomorrow.