A main oscillation fiber amplification (MOFA) system is proposed, and the increasing of pulse signal peak power and the broadening of repetition frequency modulation range of the proposed system are studied experimentally. The electrical pulse modulation and the acousto-optic modulation are employed to modulate optical pulse sequence. Amplified spontaneous emission effect and self-excited oscillation of semiconductor seed source, which are generated in the process of small signal amplifying, are suppressed. The limitation of seed source to the repetition frequency of the proposed system is eliminated, and the repetition frequency can be tuned continuously in the range of 1~200 kHz. In the process of optical pulse modulation with the acousto-optic modulator, the acousto-optic Q-switch can effectively suppress the spectral broadening caused by self-phase modulation. The rod-like photonic crystal fiber with large mode field is used as the gain medium to increase the peak power of optical pulse and the beam quality of output light, and the nonlinear effect caused by high peak power is suppressed. Ultimately, the stable optical pulse output with the pulse width of 1.12 ns and the peak power of 2.01 MW is achieved, the average power is more than 56 W, and the beam quality of the output light is close to the diffraction limit (M2x=1.49, M2y=1.54).