Considering the time delay in different hohlraum wall positions caused by oblique incidence, the spatio-temporal optical field distribution characteristics of a hohlraum wall, especially during the rising edge of a flat-topped pulse, is simulated by a fast Fourier transform method together with chromatography. Results demonstrate that beam propagation along the hohlraum wall is a push-broom process with complex dynamic spatial–temporal evolution. In the first few picoseconds, the optical intensity of the front position increases rapidly, while that of the rear position is relatively weak. The ratio R of the optical intensity during the rising edge is smaller than that of the steady state. R gradually increases and finally tends to the value of the steady state with time. Calculation also shows that, with shorter total width of the rising edge, R of the optical field decreases and the difference compared to the steady state becomes larger. The evolution is more severe with smaller angle of inclination.