I ran into a similar situation today after some partition resizing and fortunately got my problem fixed (by repairing MyDocs partition, instead of reflash which I tried first out of laziness).
After that I think I can comprehend to OPs problem and will try to explain it.

This will probably not help ducanhtian, but maybe some more experienced users can catch up.

His problem is: flasher only recognizes the N900 for half a second and then immediately jumps to u-boot.

On my device after starting flasher and the usual “no device recognized”, if now inserting the battery while pressing 'u', flasher shows “booting into flash….”, but shortly after that again shows “ no device ..” and the n900 switched to u-boot.

Tried many different ways, by changing order of cable and battery insertion, or f.e. holding 'u' all the time, but could not get the n900 into flashing mode. Instead u-boot always took over.