"Naked" ferroferric-oxide nanoparticles (FONPs) synthesized by a femtosecond laser ablation on a bulk stainless steel in liquid were applied to the Nd: YVO4 laser to achieve passive Q-switched pulse laser output. Without the pollution of ligand, the inherent light characteristic of "naked" FONPs was unaffected. The analysis of the morphological characteristics, dominant chemical elements, and phase composition of the FONPs showed that they were mainly composed of Fe3O4, which was spherical with an average diameter of 40 nm. The electron transition and orbital splitting of the iron element's octahedral center position under the laser-driven were considered the primary mechanisms of saturable absorption of Fe3O4 nanoparticles.