We report on a high power mode-locked femtosecond fiber laser oscillator with flexible double-cladding (DC) ytterbium-doped fiber (Yb-fiber) as gain medium and a fused fiber combiner as a pump coupling scheme, based on the nonlinear polarization evolution effect in a ring cavity configuration with a dual-stage birefringent plate as the spectral filter. Laser pulses with a pulse energy as high as 122 nJ and compressible pulse duration as short as 102 fs at 1041 mn center wavelength could been obtained. The experimentally recorded power fluctuations were better than 0.42% RMS and 0.40% RMS in 3 h under 5 W and 9 W output power, respectively. The measured beam quality factors M-2 were 1.3 in both the horizontal and perpendicular directions. To the best of our knowledge, this is the highest output power generated from a flexible DC Yb-fiber-based ultrashort pulse mode-locking fiber laser oscillator pumped with a fusion spliced combiner.