Considering pump depletion, walk-off effect, dispersion and nonlinear effect on pulses propagating inside optical fibers, amplification of intense pulse signal in fiber optical parametric amplifiers (FOPA) are numerically studied by solving the coupled nonlinear Schrdinger equations with split-step Fourier method. Calculation results showed that, except amplitude amplification and temporal width broadening, a very interesting phenomenon occurred that is one signal pulse splitting into two sub-pulses. Further, even if in the case of continous wave pumped, walk-off does have distinct effect on the amplification of strong pulse signal, leading to amplitude difference for two sub-pulses and pulse shift. At last, the unique strong pulse signal amplification characteristics are finely explained by saturated gain properties of FOPA, and the potential applications of pulse splitting phenomenon in pulse compression, signal sampling and time-division multiplex system are pointed out as well.