A dual-wavelength passively Q-switched Yb∶GdYSiO5 laser based on a WS2 saturable absorber mirror was proposed. The WS2 nanoplatelets solution was coated on a BK7 glass substrate with high-reflective-index thin polymer to be a WS2 saturable absorber mirror for starting Q-switching operation. By tuning the resonant cavity and selecting a suitable pumping power, the stable Q-switched laser pulses at 1051 nm and 1091 nm were simultaneously generated with a 976-nm fiber-coupled diode laser as pump source. The laser pulses had a pulse duration of 8.4 μs, a repetition rate of 2.9 kHz, and an average output power of 125 mW, respectively.