At last, we use the plugin "gridSplitter" in qgis to solve this.
basically, the "gridSplitter" plugin use gdalwarp to cut a tile from img,
it can speicfiy two argument: srcnodata and dstnodata, in my img,
set srcnodata to 0, and dstnodata 100 to set background to gray.
if blue, set dstnodata to "0 0 255".