We demonstrate a passively Q-switched Yb3+/Er3+ co-doped fiber (YEDF) laser using a filmy sodium carboxymethylcellulose (NaCMC)-based Cu1.8S nanocrystals (NCs) saturable absorber (SA). Cu1.8S NCs SA exhibit a broad absorption band from 600 nm to more than 2500 nm. By placing Cu1.8S NCs SA into a YEDF laser cavity, stable passively Q-switched laser with a central wavelength of ∼1567.2 nm was achieved at a threshold pump power of ∼1.4 W. On gradually increasing the pump power from 1.4 W to 5.6 W, the repetition rate of Q-switched laser increases from 16.6 kHz to 51.14 kHz and the pulse duration decreases from 8.7 μs to 2 μs. Particularly, we measure the output power of Q-switched lasers based on two types of plasmonic materials, Cu1.8S NCs and gold nanorods (GNRs). The maximum output power of the Q-switched laser based on Cu1.8S NCs SA is 3-4 times higher than that based on GNRs SA owing to weak photothermal effect of Cu1.8S NCs. These results show that Cu1.8S NCs are promising SAs for constructing high power pulse lasers.