The periodically poled LiTaO₃ frequency-doubling the broad-band and quasi-cw laser of seeded Ytterbium-doped double-clad fiber amplifier have been investigated experimentally and theoretically. PPLT is 40 mm long and 7.67 μm in period. The central wavelength of fundamental laser is 1.064 μm and 6 nm in bandwidth. Applying the quasi-phase matching theory and the character of fundamental laser spectrum,the relation of second harmonic power with the temperation was explained. The frequency-doubling efficiency of 1.8% is obtained when the broad band laser power is 2.2 W.