Based on the index method of low-frequency gravitational wave, diagnosis and forecast on the development of southwest vortex, intensity and area of heavy rain are discussed in this paper. The main results are as follow: (1) Development of SW vortex and heavy rain caused by SW vortex in the future 24 hours can be forecasted by the index of low-frequency gravitational wave (C_p^* and C_i^*). (2) There is correspondent relation between the variation of index of low-frequency gravitational wave with the time and development of SW vortex. (3) The values of index of low-frequency gravitational wave is also relating to the intensity of heavy rain proceed from SW vortex. (4) The area of heavy rain usually occurs in maximum negative areas of the index C_p^*, C_i^* and maximum gradient areas of C_p^* isoline.