Temporal and spatial optical field distributions of a stepped pulse on the hohlraum wall are numerically analyzed using fast-Fourier-transform method combined with chromatographic theory. Changes in the rising edge of the stepped pulse caused by spatio-temporal effect are systematically studied. Results demonstrate that a certain time difference exists for the laser to reach the hohlraum wall because of oblique incidence, which causes distortion to the stepped pulse, especially to the rising edge. Influences of the incident angle, focal length, and width of the input rising edge on distortion degree of the stepped pulse are also discussed. This letter provides some guidance for targeting pulse shaping and synchronization.