On August 20, 2019, many catastrophic debris flows broke out in meizoseismal area of Wenchuan M s 8.0 earthquake in China under the influence of continuous heavy rainfall. This paper takes the Chutou gully debris flow event occurred on August 20, 2019 as an example. The 3-D numerical simulation software, RAMMS, is used to back calculate the event and predict the future hazard. Coulomb and viscous turbulent friction, μ and ξ are calibrated in RAMMS. Numerical simulation reveals the movement process of Chutou gully from the aspects of flow depth, velocity, discharge, and run-out solid materials volume. The simulation results show that more than two-thirds of the solid materials are still deposited in the main channel, which could provide material basis for the re-occurrence of high hazard debris flow. In addition, based on the intensity and probability of debris flow, the hazard of debris flow is divided into high, middle, and low degrees. According to the simulation results and hazard assessment model, the hazard map of Chutou gully debris flow in various rainstorm return periods (20, 50, 100, and 200 years) is established, which can provide guidance for the future land-use planning and debris flow prevention works.