In order to study the amplification and propagation characteristics of optical pulse in ytterbium-doped fiber amplifier, the propagation equation of optical pulse in distributed fiber amplifier is established. Using split-step Fourier transform method, the propagation state of optical pulse is simulated, and influence of frequency detuning on characteristics of optical pulse is discussed especially. The results show that optical pulse is amplified and broadened simultaneously in ytterbium-doped fiber amplifier, so it has a deformed profile and a broadening power spectrum. For the case of frequency detuning, the amplification of optical pulse has declined and symmetry of the pulse dispears. If the center frequency of optical pulse deviates from the gain peak frequency in different directions, the deformation law of pulse is different. Therefore, the influence of dispersion and nonlinear effect of fiber amplifier and frequency detuning on the propagation characteristics of optical pulse should be considered in the design of amplification system.