As a new IV-V group semiconductor, germanium-diarsenide (GeAs 2 ) compounds have attracted considerable attention due to their outstanding optical and electrical properties, thickness-dependent bandgap, in-plane anisotropy, and excellent optical absorption. However, the potential of GeAs 2 in the field of ultrafast and ultranarrow fiber laser has not been studied. In this article, a high-quality GeAs 2 nanosheets saturable absorber (SA) is successfully prepared by liquid-phase exfoliation. The nonlinear optical characteristics of GeAs 2 nanosheets have been investigated based on a balanced twin-detector measurement system. The modulation depth, nonsaturable loss, and saturation intensity are measured to be 5.2%, 24%, and 1.23 GW cm −2 , respectively. GeAs 2 has been successfully applied as an SA in an ultrafast and singlefrequency fiber laser. A stable mode-locked laser pulses operation with a duration as short as 371 fs and a repetition rate of 8.19 MHz at a wavelength of 1560 nm is achieved. Moreover, ultranarrow fiber lasers with a high signalto-noise ratio of 80 dB and a linewidth of ≈678 Hz are obtained. The findings validate that 2D GeAs 2 can be used as an SA and has promising applications in ultrafast and ultranarrow photonics.