well,i'm an undergraduate. -> Well, first of all, I'm an undergraduate student.

When you use 'well', it kind of has a nuance of you answering a question hesitatingly or you are answering something or responding to something hesitatingly. If you start out with 'well' when you're just speaking, it's better to stick on a 'first' or 'starting out' or something to tell your readers that you're going to be speaking to them.

i'm lemon,from hebei province ,north of the China. -> I'm Lemon from the Hebei province in northern China.

i came to wuhan to study for almost four years. -> I came to Wuhan to study and have been here for almost four years now.

This was a little hard for me to try to keep it the same as you wanted it. If you mean that you go to Wuhan everyday from here you live then you can say 'I've gone to Wuhan to study for almost four years' but if you moved there then my suggestion is better. Your tenses don't really agree.

i'm that kind of person who is optimistic,aggressive and diligent girl . -> I'm the kind of (person/girl) who is optimistic, agressive and diligent.

Using 'who is blahblah' means that you are already describing a noun that came before 'who' so adding girl at the end just has two nouns for one adjective. So instead you can just use person or girl (whatever you wish) in the front.

English is my favourite,since i was a junior student ,i just loved it. -> English has been my favorite since I was a junior in high school. I just love it.

I assume junior student = junior in high school because you said you're an undergraduate which implies college. Also, using 'loved' means that you don't anymore but I think that you still do. XD

But, I still have many weaknesses like speaking,vocabulary,and so on. -> But I still have many weaknesses like speaking, vocabulary, and so on.

so,i wish that i can make friends and learn more in order to improve myself.thanks. -> So I wish that I can make friends and learn more about English in order to improve myself. Thanks.

I'm not sure if you meant learn more about things in general or just learn more about English but I assumed it would be about English since it is an entry about English. But you know what they say when you assume...

Anyways, you did a good job! Have fun learning English! =)