Benvolio cautions Mercutio that his making fun of Romeo is likely to anger him. 

And if he hear thee, thou wilt anger him.

But Mercutio disagrees; his conjuring, he says, would not anger Romeo in the least. He then proceeds to describe the kind of conjuring which would anger Romeo.

Twould anger him to raise a spirit in his mistress' circle,

You may have noticed that Fairies and witches are often portrayed as holding hands and dancing in a circle. That’s because they are creating a magic circle where they can conjure a spirit to rise up. 

But when Mercutio refers to Romeo's mistress’ circle, he’s thinking about another kind of circle where something may “rise up.” What would really irritate Romeo is if Mercutio conjured up some stranger’s spirit in his mistress’ circle ...

a spirit in his mistress' circle of some strange nature

and left it standing there until she "laid it and conjured it down."

Well, yes, I could certainly see how that might irritate Romeo. But, Mercutio’s not going to conjure up some stranger; oh no, it’s only Romeo who he will "raise up" for his mistress’ enjoyment.

in his mistress' name I conjure only but to raise up him