A single-stage femtosecond laser direct amplification system with all polarization-maintaining, double-cladding, Yb-doped large-mode-area photonic crystal fiber (LMA-PCF) is constructed. The photonic crystal fiber (PCF) oscillator operates in soliton mode-locking regime, and the PCF amplifier is employed in nonlinear amplification. The all-PCF single-stage amplifier directly generates a high-average power of 34 W and a pulse width of 50 fs at the repetition rate of 42 MHz, corresponding to pulse energy of 0.8 μJ and peak power of 16.2 MW.