An ytterbium-doped single polarization large-mode-area photonic crystal fiber laser amplifier is demonstrated, which can deliver average power of 16 W at a repetition rate of 50 MHz, corresponding to 320 nJ pulse energy. The length of the fiber is 3.5 m, and the pulse could be shorter since the spectrum is further broadened due to the SPM. The pulse duration is compressed to 85 fs after passing through a grating pair. The oscillator and amplifier of the system are composed of the same single polarization large-mode-area photonic crystal fiber, resulting in better environmental stability and compactness.