The three-energy modelsimulated with photocycle characteristics is used to explain bacteriorhodopsin’s 3-order nonlinearsusceptibility and its time response, which was measured with four-wave mixing technique.The saturation density and length of excitons in bacteriorhodopsin are obtained withfitting to the phase-space filling model. The possible mechanism for generating the 3rdnonlinear optical susceptibility χ(3) and response time are discussed.