The nonlinear optical properties of F-P cavity with a second order nonlinear crystal inside are discussed according to the effective third order susceptibility, which is formularized by the cascaded second-order processes in nonlinear crystal in small signal conversion condition. The analysis shows that the power density passing through the nonlinear crystal is several ten times higher than input power density. Enhanced nonlinear phase shift is changed with wave vector mismatch. The maximum phase shift of fundamental beam is observed when the wave vector mismatch is near 2π. This phase shift is changed with input power density, second order susceptibility and length of nonlinear crystal.