  Besides the original seeded undulator line, in the Soft X-ray free-electron
laser (SXFEL) user facility at Shanghai, a second undulator line based on
self-amplified spontaneous emission is proposed to achieve 2 nm laser pulse
with extremely high brightness. In this paper, the beam energy deviation
induced by the undulator wakefields is numerically obtained, and it is verified
to have a good agreement between 3D and 2D simulation results. The beam energy
loss along the undulator degrades the expected FEL output performance. Impact
of wakefields on pulse energy, radiation power and spectrum is discussed, as
well as the benefits of compensation obtained with a taper in the undulator
field. And using the planned SXFEL diagnostic, a longitudinal wakefields
measurement experiment is proposed and simulated.
