By variational approach, the characteristic of Z-scan in thick media with high order nonlinearity is theoretically analyzed. In comparison with the result of Z-scan only considering the third-order nonlinearity. It is found out that the fifth-order nonlinearity can not be neglected when the media has large fifth-order nonlineartiy or the intensity of laser is more powerful. These results are useful in searching high performance optical nonlinear materials and designing optical limiter.