The present invention is applicable to the field of fiber laser technologies. In the present invention, two fiber lasers of different bands are used as seed sources to form a dual-band fiber laser that outputs beams of two bands simultaneously, and the dual-band fiber laser is used to pump the cascaded evolving assemblies. Specifically, when the laser beams of the two bands go through a first-stage evolving assembly, the laser of one band is evolved into a visible-to-near-infrared supercontinuum, while the laser of the other band is evolved into an ultra-short pulse; the visible-to-near-infrared supercontinuum and the ultra-short pulse are coupled and enter a second-stage evolving assembly; the ultra-short pulse is further evolved into a near-infrared-to-mid-infrared supercontinuum, in which the visible-to-near-infrared supercontinuum generated at the previous stage is transmitted in a low loss manner; and an ultra-wideband supercontinuum covering visible, near-infrared, and mid-infrared bands is finally output from cascaded fibers.