The theoretical and experimental study of an Yb-doped double clad large mode area photonic crystal fiber laser is reported. This fiber laser system uses one facet of the fiber directly as a cavity mirror， which is fine-polished to 0° angle after collapsing. Lasing is obtained with the feedback of this facet， and mode-locking is achieved by using a semiconductor saturable absorber as another cavity mirror and adding a grating-pair as the spectral filter. By adjusting the filtering conditions， continuous tuning from wide filtering condition to narrow filtering condition is achieved. Under the wide filtering condition， the system generates femtosecond pulses， the highest average power of which is 2.2 W， corresponding to 29.3 nJ pulse energy and 367 fs pulse duration; under the narrow filtering condition， it generates picosecond pulses， the highest average power of which is 4 W， corresponding to 53.3 nJ pulse energy and 1.14 ps pulse duration. In numerical simulation， the evolution and cavity-position-related characteristics of pulses are studied by using split-step Fourier method.