Abstract: The applying and modifying study on the Lagrange method is carried out. The results show that the Lagrange method on the isentropic surface gives the best effect, and the variation of the material contour can be studied more finely using the modified Lagrange method. Based on the experiments, the transport of the water vapor in the case during the period of Changjiang-Huaihe Basin Meiyu in July 1991 is diagnosed. The analyses show that the vapor is mainly transported from the South China Sea and the Bay of Bengal, and the latter is stronger.