Beam smoothing by spectral dispersion (SSD) technology is widely adopted in larger laser fusion facilities. Using intensity distribution of SSD beam at its focus plane as boundary condition of laser plasma simulation code LAP3D, we simulate the propagation of SSD beam in large scale hohlraum underdense plasma. High efficiency parallel calculation of intensity distribution of a SSD beam at its focus plane is accomplished with improved algorithm, and then the simulation of entire beam propagation can be processed. The effect of modulation frequency of SSD beam on beam propagation is analyzed.