It looks superficially like Dark-sided but it's not right structurally - the bill and head are too large and the wings are nowhere near long enough. How about Grey-streaked?
I agree with Andy (who undoubtedly has more field experience of Siberian and Grey streaked) and may also add that the fringing on the wing coverts and tertials appears fairly clean and whitish - I understand that this is also a supporting feature of Grey streaked over Siberian too.
Here is an old, but hopefully still relevant paper on the ID of the three "brown" flycatchers.
It looks like Dark-sided to me.
Note that the birds still has a lot of juvenile spotting on the upperparts (lesser coverts, scapulars, rump and head). Dark-sided is the only one of the three to regularly migrate through China in juvenile plumage, although the other two will apparently also do so occasionally.
The underparts are quite dark and seem mottled, not as clearly streaked as they should be on Grey-streaked. The head is also quite dark, with a very dark malar stripe contrasting with paler submoustachial. The pale fringes to the greater coverts and tertials aren't a problem for Dark-sided, especially given the strong lighting from above. Eye-ring shape is hard to judge but does seem to be broader behind the eye, as is typical on Dark-sided. The lores are not as pale as I'd expect on Grey-streaked.
Structurally it's quite small-headed and small-billed. The wings are quite long (I would say they're too long for Asian Brown). My main issue is that the tail looks a bit longer than normal for Dark-sided (but I wonder if it is angled slightly towards the camera).
Note the large white spots on the rump of this bird - I don't know if this is typical of juvenile Grey-streaked.