Abstract: Since 1998, Helankou Gully has experienced frequent flash floods and debris flows due to extremely heavy rainstorm, which caused significant damage to the rock art in the gully area in 2016.Therefore, flash flood forecasting is an important part of Helan Mountain rock art protection work. This paper proposes a probabilistic forecasting method for calculating peak discharge by combining meteorological warning information, hydrological conditions and probabilistic methods. According to the flood discharge capacity of Helankou Gully, if there is rainfall of over 100 mm within 3 hours, the probability of causing flash flood disasters is 99%. For the common rainstorm early warning of "more than 50 mm rainfall within 3 hours", the probability of flash flood causing disasters is 37%.The research results can provide early warning time for the protection of Helan Mountain rock art and offer an probability prediction method for similar small watershed flash floods.