I am going to study for one year at Yunnan University.
The studies will begin in september.
Now I am searching for a fitting accommodation.
Can you give me a tip? Is it better to live on the campus or to
hire a flat? What's is the best way to find a vacant flat there?
Sure live out of the campus as the campus dorm conditions is miserable.If i were you I would not even choose Yunnan university to study Chinese. Almost teachers there unqualified with accents and never speak pure Chinese. Traditional ways of teaching and boring classes.
It depends totally on your budget. If you wanna live really really cheaply the dorms are okay (at least in Yunnan Normal University), and you'll really save an enormous amount of money - no other accomodation anywhere in Kunming will be even nearly as cheap. This is assuming you live in the chinese/asian dorms where you share a room with people, if Yunda has dorms intented for westerners they'll likely be horribly overpriced.
The other options are to either get a room in a flat, or a whole flat for yourself. Small flats are hard to come by in Kunming, so probably sharing an apartment with somebody or renting a room in somebody's apartment are more realistic options (unless you're rich). If you want a cheap, small flat for yourself you'll likely have to live in a shabby neighborhood far from the city center.
Come august, there'll be loads of room and apartment ads here on GoKunming, so finding a place won't be difficult, but most of the rooms/flats on this site are very expensive for what you get; a room in central Kunming really don't need to cost more than 6-800 yuan per month. So be careful with what you chose; it might be better to go here first (cheap hostels and dirt cheap shabby hotels are very easy to find) and then look for a room/an apartment.