I am looking for a way to spend half the year in Kunming and half in Europe for the next few years, and if possible not to have to look for a new apartment each time I come back. Does anyone have experience sub-letting their apartment for a few months at a time? Is it easy to find tenants and how strict do Kunming landlords tend to be about this?

Short term it is often done, but hidden from the landlord. I am not sure a landlord would be happy, as technically sub-letting unofficially is illegal. The landlord is also legally responsible for the tenants and to some degree their activities.
Any foreign tenant should register with the local police, but would need a rental agreement in order to do this.

I also believe that any non-Kunming citizen also needs to register with local police for a temporary ID card, and they too would need a rental agreement.

If by half a year in each place you mean 6 months at a time, then you might find this difficult. If only 2 months at a time, you might have difficulty finding tenants each time.

An alternative might be a flat share, and come to an arrangement about rent when you are not there.

Also you should consider whether (combined with your other income, if any) this creates some tax issues in either locations.

You could put yourself in position to not only having to hide the sub-letting itself from the landlord and authorities, but also the money.

Breaking law (as tigertiger mentioned) is one thing, but making money by breaking it would make it much worse.

Alternative to sub-letting would be to negotiate with landlord that you find another tenant for 6 months, during which the rentral contract would be in that person's name, and then return back to you. You'd need to make paperwork to be sure that you get the apartment back though, and depending with your relationship with the landlord, be ready to enforce it.

Yeah I agree with @tiger, you would have to get the rental agreement with your name on it, in order to get the registration of residence done at the local police. So you have to be the one to sign the paperwork, or at least you need to be one of the tenants on the lease contract. There is no way around this, not without false documents. Sub-letting is on the other hand easier to deal with, as long as you can find the right tenant who fits your timetable.

I've taken several apartment rents since I came back to China in 2015, a few in Hangzhou and now the first one in Kunming, and in my experience, I don't think the landlord cares too much about who is living in the property as long as everything in the property is taken good care of. Sometimes you even met landlord who lives in a completely different area of the country. So there will only be the extreme cases in which the landlord lives in the neibourghhood, and he/she happens to be that cautious landlord who cares so much about the property that he/she wants to sneak around and see what's going on once a while, otherwise you will most likely be free to sub-let the property to a third party. Just be careful who you sub-let the lease to.

Air BandB is always an option.

Thanks for these replies,

lots of useful input.

Sounds like plenty of hassle...

Airbnb would be easier and I guess less likely to cause problems with the landlord, although also not so legal, right? Anyone have experience renting out a place in Kunming on Airbnb? Any trouble?

Nothing ventured nothing gained.

Yeah @sarawati I think your best shot would be finding a property on Airbnb, there should be plenty of landlords there willing to do 6 months rent, already pretty long-term for them. The price will end up higher, but considering the difficulty finding a tenant who exactly fits your timetable, I think the price will eventually be evened out.

If the airbnb landlord agrees, maybe you can go to a local real estate agency together and see if they are willing to issue a lease contract. If you pay the right money, I think they will do it for you. When you rent a property from them, in Kunming usually they charge a month rental apart from the rental you need to pay the landlord, so you have an idea of how much you are expected to pay.

If you do this each time you come back to Kunming, I think you will be good legislation-wise, as every time you will have the paperwork ready for the residence registration, nothing illegal about that :P