Based on nonlinear Schrdinger equation in optical fiber, the factor of distortion of pulse (DIS) is defined to evaluate the distortion of the pulse. The relation among the DIS, critical length of propagation, critical power and initial chirp of the pulse are numerically simulated and analyzed when only dispersion and self-phase modulation are considered. The results show that the distortion of positive chirped pulse is less than that of negative chirped pulse for given peak power. For given initial chirp, the critical length of pulse decreases with increasing of peak power of input pulse, and critical length of different initial chirp pulses coincides along with increasing of peak power of input pulse. When the length of propagation is fixed, critical power is linear with initial chirp, meanwhile the fluctuation of chirp have more effect on the positive chirped pulse.