  In this paper, we propose to use the LWFA as the preinjector for XFEL driver
linac. We can use LWFA to produce the femtosecond electron beam. The peak
current of the produced beam can reach tens of $kA$ and is high enough to drive
XFEL so that the present bunch compressing technique can be deserted. The
output optical pusle length can be as short as $ 1 fs $.
