The writing of double line waveguide in Yb3+ doped yttrium aluminum garnet (Yb:YAG) crystal is studied by using titanium sapphire laser with the pulse width of 160 fs, central wavelength of 775 nm and repetition frequency of 50 kHz. The phenomenon of polarization guiding is discovered in the waveguide. The laser with polarization parallel to the direction of double line can be guided, but the laser with polarization perpendicular to the direction of double line cannot be guided. The effects of written parameters, such as double line separation, laser pulse energy and writing speed, on the formation of the waveguide are analyzed. Experimental results show that the waveguide has the good property of guiding under the writing conditions of double line separation of 30 μm, writing speed of 400 μm/s and pulse energy of 5.0 μJ. Bivariate distribution of the waveguide refractive index is reconstructed by near-field mode and the maximum refractive index change is 1.8×10-4. A continuous waveguide laser with a wavelength of 1030.5 nm is obtained and the output power is 4.7 mW.