A numerical wave model based on the modified fourth-order nonlinear Schroe dinger equation (mNLSE) in deep water was developed to simulate the formation of freak waves and a standard split-step, pseudo-spectral method was used to solve the equation.The validation of the model is firstly verified, then the simulation of freak waves was performed by changing sideband conditions, and the variation of wave energy was also analyzed in the evolution.The results indicate that Benjamin-Feir instability (sideband instability) is an important mechanism for freak wave formation.