He is not stupid as you think

Yes, except, although this is a common colloquial way to use the verb "to think" the correct verb in this context is "to believe". "To think" has to do with cognative process.

Are you trying to say that he is more stupid than people belive?

He is not as stupid as you think [he is].

He is not stupid, despite what people think.

Your are not completing the comparison in your first examples. You need to use the structure 'as X as'.

a/ He is not as stupid as you think
b/He is not as stupid as you think he is
c/he is not as intelligent as most people think
d/He is not as intelligent as most people think he is

No . when I say "he is not stupid as you think" I mean he is not stupid at all and you cannot easily cheat him

TuongvanNo . when I say "he is not stupid as you think" I mean he is not stupid at all and you cannot easily cheat him

No. Use the examples Nona and I gave.

He is not as stupid as you think he is.

He is not stupid, despite what you think.

a/ He is not as stupid as you think
b/He is not as stupid as you think he is
c/he is not as intelligent as most people think
d/He is not as intelligent as most people think he is
e/He is more intelligent/stupid than most people think (he is)
All OK as corrected

Grammar GeekAre you trying to say that he is more stupid than people belive?

Yes. What's "believing" got to do with "thinking"? Just my opinion.

Thank you very much Grammar Geek,Nona The Brit ,and Califjim.

Hi Huevos

The use of the verb 'think' is fine in the sentence. Using the verb 'think' to refer to an opinion or belief is quite standard.
http://dictionary.cambridge.org/define.asp?key=82584&dict=CALD

Native speakers of American English won't find nuances of meaning between "Despite what you think, he's not stupd" and "Despite what you believe, he's not stupid." Nor will they find differences between "He's not as stupid as you think he is" and "He's not as stupid as you believe him to be."