It is well known that global climate change will lead to more frequent extreme precipitation events. Current studies mostly focus on the structural damage caused by water to immovable cultural relics and often ignore the effects of prolonged immersion in the hygrothermal environment. Therefore, it is critical to assess the environmental impact of flood hazards caused by extreme precipitation events for heritage conservation. Cliff tombs in Sichuan have a long history but are highly vulnerable to rising water levels due to their riverside location. In this study, we established a hygrothermal simulation model for the tomb of Huangsan Cliff and explored the effects of different flood duration on the tomb. The results indicated that the relative humidity on the surface of the cliff tomb would remain consistently high for a considerable duration after water immersion. There was a discernible trend that the closer to the inner chamber, the longer the humidity would return to normal levels. Moreover, the moisture changes trends between points on the ground and the ceiling show distinctly different trends. Additionally, analyzing surface degradation requires considering its initial temperature and humidity conditions.