To research the mechanism of high power supercontinuum generation by sub-nanosecond pulse pumping photonic crystal fiber (PCF),a 1.8 m PCF is pumped by 570 ps pulses from a Yb-doped mode-locked fiber laser,and it puts out SC (supercontinuum) with 1.15 W power and 750 nm spectral coverage.It is verified that when SC is generated by sub-nanosecond pulse pumping 1.8 m PCF,the modulation instability (MI) takes an important part in expanding spectrum via comparing and analyzing the experiment with simulation.After researching the change of SC under different pump power,it is found that the power of SC increases and the spectral coverage expands with the pump power.Under watt power output,it still does not reach saturated section,and be probable to get higher power and broader spectrum.