HI 高枫
No, "I have been considering the thesis is excellent?" is not grammatically correct.
It is possible for the two words, consider and think, to have the same meaning, but the sentences are constructed slightly differently.
a) to 'think about' something can be to 'consider' something
b) to 'think that' something can be to 'consider s/t to be' something
examples:
a) I have been considering this thesis; it is excellent
a) I have been thinking about this thesis; it is excellent
b) I think this thesis is excellent
b) I consider this thesis to be excellent
a) and b) together: I have considered this thesis and I think it is excellent.
a) and b) together: I have been thinking about this thesis and I consider it to be excellent.
I am sure there are plenty of other ways to say it, but hopefully this highlights the difference.
There are other meanings for both 'think' and consider', but they are not really relevant to the
sentence you asked about.