Control of exhaust emission and combustion efficiency depend directly on the atomization of the liquid. The research on the atomizing performance of fuel nozzle is certainly important. A model using VOF-LES method has been built to investigate sheet breakup behavior of the main circuit of a pressure swirl atomizer. LES which takes into account of turbulence of fuel and air, has been used to simulate the phenomenon of sheet breakup. Vivid gas-liquid interface, instability wave and primary breakup phenomenon have been captured. The patterns of the sheet breaking-up change with different fuel flux. The mechanism of sheet breakup has been analyzed by combining variations of velocity and turbulent kinetic energy with Kelvin-Helmholtz instability theory and turbulence on circumference. The breakup length captured by simulation agrees well with the results of the semi-empirical formulas.