  X-ray free-electron lasers (XFELs) are cutting-edge scientific instruments
for a wide range of disciplines. Conventionally, the narrow bandwidth is
pursued in an XFEL. However, in recent years, the large-bandwidth XFEL
operation schemes are proposed for X-ray spectroscopy and X-ray
crystallography, in which over-compression is a promising scheme to produce
broad-bandwidth XFEL pulses through increasing the electron beam energy chirp.
In this paper, combining with the beam yaw correction to overcome the
transverse slice misalignment caused by the coherent synchrotron radiation,
finding out the over-compression working point of the linac is treated as a
many-objective (having four or more objectives) optimization problem, thus the
non-dominated sorting genetic algorithm III is applied to the beam dynamic
optimization for the first time. Start-to-end simulations demonstrate a full
bandwidth of 4.6% for Shanghai soft x-ray free-electron laser user facility.
