Thank you for RUWright's informative comment!
I am grateful to all suggestion. Although the first sense of this bird is not a yellow-browed warbler to me, it comes to be much pointed to yellow-browed...
For the Hume's yellow-Browed warbler, I am totally unfamiliar with it! Thanks for telling more information for this bird=]
I may conclude that it might be a yellow-browed warbler.
On the field, it seemed quite large, but may be the weather is so cold. Also, it jumped on the field but not trees. Finally, the black tone of legs and bill were strange. To sum up of these features, I am pluzzed for its ID, but now, I think my guesses of Bylth's leaf warbler and greenish warbler were far from acceptable guesses. I am sorry about that
Many thanks again!