Abstract: Based on the methods of Fourier decomposition, correlation analysis and Liang-Kleeman information flow, the interaction between intensity and rainfall of Typhoon Rammasun (1409) is studied using the best track data of Shanghai Typhoon Institute (STI) of China Meteorological Administration (CMA), Tropical Rainfall Measuring Mission (TRMM) satellite 3B42 rainfall estimation data of National Aeronautics and Space Administration (NASA) and ERA5 reanalysis data of European Centre for Medium-Range Weather Forecasts (ECMWF) with 0.25°×0.25° grids. The results show that the rainfall of Typhoon Rammasun (1409) has obvious characteristics of asymmetry, and it is mainly located in the west side of the typhoon center. The rainfall of Typhoon Rammasun (1409) increases significantly twice during the whole life cycle, which corresponds to the intensification period. The information flow analysis shows that the rainfall of Typhoon Rammasun (1409) is affected by the intensity of typhoon itself, whereas the rainfall can also feedback on the latter. Compared with the influence of typhoon intensity on rainfall, the information flow from rainfall to typhoon intensity decreases by nearly an order of magnitude, indicating that the typhoon intensity plays a dominant role in the interaction relationship. The possible mechanism by which the typhoon intensity affects rainfall are analyzed by diagnosing the water vapor and dynamic conditions respectively. In terms of water vapor conditions, the convergence area of vertical integral of moisture flux corresponds well to the rainfall. The intensification (reduction) of Typhoon Rammasun (1409) partly results in the enhanced convergence (divergence) of vertical integral of moisture flux in the southwest of the typhoon center, which brings more (less) rainfall in this region. In addition, the South China Sea and the Western Pacific water vapor transport channel have obvious response to the changes of Typhoon Rammasun (1409) intensity. In terms of dynamic conditions, the strong center of vertical helicity in the middle and lower layers is mainly located in the west side of the typhoon center, indicating the rainfall area. The intensification (reduction) of Typhoon Rammasun (1409) leads to the increase (decrease) of absolute vertical helicity on the west side of the typhoon center, thus promotes (inhibits) the development of upward movement to a certain extent, resulting in more (less) water vapor condensation and rainfall.