  In free electron laser facilities, almost every kind of device will generate
wakefield when an electron beam passes through it. Most of the wakefields are
undesired and have a negative effect on the electron beam, which means a
decrease of FEL performance. As for the SXFEL test facility, the sophisticated
layout and the cumulative effect of such a long undulator section lead to an
obvious wakefield, which is strong enough that can not be ignored. Based on two
deflecting cavities at the entrance and the exit of the undulator section with
corresponding profile monitors, we measured the wakefield of the undulator
section. In this paper, we give the theoretical and simulation results of
resistive wall wakefields which agree well with each other. In addition, the
experimental and the simulation results of the overall undulator wakefield are
given showing small difference. In order to explore the impact of this
wakefield on FEL lasing, we give the simulation results of FEL with and without
wakefield for comparison. There is almost no impact on 44 nm FEL in stage-1 of
cascaded EEHG-HGHG mode, while the impact on 8.8 nm FEL in stage-2 becomes
critical decreasing the pulse energy and peak power by 42% and 27% and
broadening the bandwidth.
