The generation of ultra-short pulses and the formation of supercontinuum in the sub-nanosecond pulse pumping regime have been simulated. First of all, we briefly introduced the generalized nonlinear Schrodinger equation,which could effectively describe laser propagation in the nonlinear medium. The choice of temporal resolution and the high-accuracy adaptive change of step size were secondly studied for the split-step Fourier method. The formation of supercontinuum was finally researched by gradually increasing the peak power of incident pulse. The results showed that sub-picosecond pulses could be generated from the breakup of sub-nanosecond pulse owing to modulation instability, and the higher-order nonlinear effects of ultra-short pulses, including self-steepening, intrapulse Raman scattering, the fission of higher-order soliton, lead to the formation of supercontinuum spanning from 481 nm to1 750 nm of 30 dB bandwidth.