A numerical model for nonlinear polarization rotation mode-locked fiber laser is established based on coupled wave equations and Jones matrix. The numerical results show that the power jitter of pump laser seriously affects amplitude jitter of mode-locked fiber laser. The root mean square (RMS) value of mode-locked pulse is 0.82%, and peak-to-valley value is 2.84% when random jitter of pump power is ±3%. Cavity length jitter has less effect on amplitude jitter. However, cavity length jitter leads to large timing jitter. So in order to get mode-locked pulses with very low timing jitter and amplitude jitter, pump laser power jitter and cavity length jitter must be controlled.