A dissipative-soliton mode locked laser based on Yb-doped single polarization large-mode-area photonic crystal fiber is demonstrated. The dissipative soliton dynamics is explored by numerical simulation. Compared with all normal dispersion fiber lasers, there are more pulse shaping mechanisms in a dissipative-soliton fiber laser, including the nonlinear absorption of SESAM, spectrum filtering of the chirped pulse and gain dispersion, which results in robust and stable operation. Among them, spectrum filtering plays a dominant role in mode-locking, it shapes pulse both in frequency domain and time domain and keeps the pulse duration below 1 ps in the cavity. For the first time, sub-1 ps pulse is directly generated in a dispersion compensation free cavity. The fiber laser directly generates 777 fs pulse at 1 W average power at a repetition rate of 514 MHz, corresponding to a single pulse energy of 20 nJ.