Abstract: Cytochrome P450 has been well known to play crucial roles in pesticide resistance in insects. In this study, binding sites, type and size of the driving force between cytochrome P450(CYP9A11) in Spodoptera exigua and three pesticides were investigated by AutoDock molecular docking and molecular mechanics Poisson-Boltzmann surface area(MM-PBSA), MM-PBSA free energy calculation. The results showed as follows:when CYP9A11 combined with chlorpyrifos, two hydrogen bonds and eight hydrophobic residues were formed, and the binding free energy was -3659.80 kJ/mol. Five hydrogen bonds and three hydrophobic residues were formed when CYP9A11 combined with methomyl, and the binding free energy was -470.92 kJ/mol. Seven hydrophobic residues formed when CYP9A11 combined with cypermethrin and the binding free energy was -473.44 kJ/mol. The van der Wals force was the main driving force of the docking between CYP9A11 and chlorpyrifos, methomyl and cypermethrin. The main driving force of binding to CYP9A11 was the polar solvation energy. These data could serve as references of illuminating the binding mechanisms of the above mentioned three pesticides with CYP9A11 in S. exigua.