The laser diode (LD) is used to pump the passively Q-switched dual-wavelength laser with Yb∶YAG slab crystal, and the experimental study of controllable laser output polarization is carried out. A compact V-shaped resonator is built in the experiment, and the laser output can be switched from 0° horizontal polarization to 90° vertical polarization by adjusting the inclination of the output mirror (OC). Through the experimental design, it can be concluded that the final output polarization depends on the additional phase difference in the cavity. When the additional phase difference in the cavity is π, the output polarization changes from horizontal polarization to vertical polarization. The experimental results show that when the change of OC inclination angle is about 3.6 mrad, the linear polarization is switched from horizontal polarization to vertical polarization, the optimal extinction ratio of horizontal and vertical polarization is 28.98 dB and 25.96 dB, respectively. When the pump power is 18.42 W, a balanced dual-wavelength laser output of 1.66 W and the corresponding pulse width of 42.6 ns are obtained.