The axnplification of low-repetition-rate pulse is numerically studied in Pr3+-doped fiber amplifier(PDFA). By optimizing constructional parameters, such as fiber length modal and dopant radii, the gain coefficient increases by 0. 02 dB/mW and the output pulse energy is up to 0. 02 mJ for the pump power of 100 mW. This improvement results from both making good use of Pr3+-doped fiber, and suppressing amplified spontaneous emission (ASE) and excited state absorption (ESA) of pump photons.