I'm going to Jade dragon Snow mountain in late September, will there be snow?

Yes, of course. You can even see the snow in summer.

I think it depends on how much snow you expect to see, to be honest, the snow cover can be a bit thin during summer time. But if it doesn't rain and you have a clear view, you can see the glacier, which is pretty impressive, so that more than makes up for the lack of snow, in my opinion.